NEWARK, N.J. (AP) _ Brothers Jack and Charles Colbert did a lot of washing in jail, but it wasn't clothes they were laundering, it was postage stamps, the government says.

An indictment announced Friday charges the Colberts washed cancellations from the stamps and sold them at a discount in a mail-order business they ran from the Somerset County Jail.

''They were just running a small business out of a jail cell,'' said Postal Inspector Walter A. Gilliland.

The imprisoned pair could be sentenced to 39 additional years each if convicted on charges of conspiracy, mail fraud and misuse of postage stamps, said Assistant U.S. Attorney Stuart J. Rabner.

Officials said they caught on to the scheme in March after being tipped off by potential customers.

Jack Colbert, 37, and Charles Colbert, 42, originally from Mount Vernon, N.Y., are serving 13-year federal prison sentences for 1986 convictions on mail fraud, wire fraud and conspiracy charges stemming from a toxic waste disposal scheme in New York.

The indictment says that while the brothers spent December until June in the county jail awaiting trial on state toxic waste dumping charges, they ordered canceled stamps in bulk.

In one case, they allegedly sought to buy 110 pounds of canceled stamps from a source the indictment did not specify. Bulk sales of canceled stamps are advertised in stamp-collecting magazines.

The brothers also wrote the March of Dimes asking for canceled stamps, according to the charges.

The Colberts offered to sell ''mint,'' ''ungummed'' stamps at a 75 percent discount in minimum $20 lots. The indictment did not reveal how the cancellations were cleaned from the stamps.

Gilliland would say only that corrections officers turned over two empty shampoo bottles and a partly filled plastic container of liquid laundry detergent taken from the Colberts' shared cell.

Gilliland said the men were given extra mailing privileges because of the complexity of the toxic waste charges they faced.

''I guess they kind of flim-flammed the officials at the jail, saying they were making mailings out to defense attorneys and businesses trying to formulate a defense,'' he said.

The state dumping charges were dismissed in June and the brothers have since been transferred to federal prison in Otisville, N.Y.