WASHINGTON (AP) _ Excessive radioactivity, possibly caused by the Chernobyl nuclear accident, was found in a shipment of beef extract from Brazil, the Agriculture Department says.

''We believe the contamination is related to the April 1986 nuclear accident at Chernobyl,'' Donald L. Houston, administrator of the Food Safety Inspection Service, said Friday. ''However, we don't yet know from which European country Brazil obtained the beef to make the extract.''

The shipment was caught by a USDA inspector in Newark, N.J., recently and denied entry to the United States, the department said.

''We believe this to be an isolated incident but we won't know for sure until we test additional products,'' Houston said in a statement.

Intensified testing of Brazilian meat imports has been ordered, USDA said. It said it also is asking Brazil to step up some inspections.

Houston said, however, that higher radiation levels could be expected in the extract than in the beef itself because the extract is more concentrated.

The extract consists of highly concentrated beef juices and is used mainly in soup and similar products. It comes in 56-pound containers and is not sold retail.

Houston said that the product did not represent a health hazard but that it was rejected because it exceeded the official limit of 10,000 picocuries of cesium per kilogram.

The extract registered 17,000 picocuries, he said.

Additional tests of Brazilian beef extract, including others from the plant where the contaminated shipment originated, showed no radioactivity.

USDA said that in addition to denying entry into the United States of the shipment, it is taking the following actions to prevent contaminated products from reaching consumers:

- Stepping up port-of-entry testing of all products from the Sao Paulo company where the shipment originated as well as of beef extract from all Brazilian plants.

- Requiring Brazil to test all beef extract for export to the United States.

- Attempting to locate and test extract from all plants in Brazil as well as all meat products from the Sao Paulo plant that already have entered this country.

This is the first radiation violation since the Chernyobl disaster, which has prompted USDA to require countries exposed to the fallout to certify that products they send here have less than 10,000 picocuries.

In addition, the accident spurred USDA to start radiation tests of incoming food in U.S. ports.

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WASHINGTON (AP) - A California congresswoman is urging the Environmental Protection Agency to curb imports of produce fumigated with the pesticide EDB, which has been restricted in the United States.

Rep. Barbara Boxer, D-Calif., especially cited Mexican mangoes, which under an Agriculture Department rule must be sprayed with EDB before being brought into the country to prevent import of the Mediterranean fruit fly. She said 27 countries are operating under an exception to the ban on the pesticide.

''Either EDB is a health hazard or it is not,'' she said in a statement. ''We cannot allow issues of economic impact upon other countries to dictate the safety of the American food supply. If foreign growers cannot conform to our domestic pesticide standards we should ban ... their products.''

She asked EPA Administrator Lee Thomas to revoke any exceptions that allow import of produce subjected to pesticides banned domestically. She also wrote to grocers in her district asking them to stop selling imported mangoes.

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WASHINGTON (AP) - Rep. Charles Rose, D-N.C., plans a hearing this week in his House tobacco and peanuts subcommittee on the pesticide program for domestic and imported tobacco.

Rose said in a statement that he set the hearing Thursday as a result of recent reports that cigarettes made in this country with imported tobacco and shipped to Japan contained excess residues of the herbicide dicamba.

The reports raise questions about enforcement of a 1985 law requiring importers to certify that imported tobacco does not contain excess residues of pesticides banned from domestic use on tobacco.

''The subcommittee wants to review the way in which the 1985 law has been administered,'' Rose said. ''We want to find out, among other things, why the Agriculture Department did not notify the subcommittee that a problem existed.''