LAS VEGAS, Nev. (AP) _ Prosecutors dropped charges against two of 12 defendants in a federal racketeering trial, then began questioning an informant who infiltrated Las Vegas crime operations in 1980 and 1981.

Defendants Frederick Pandolfo and Herbert Blitzstein were dropped from the trial in a surprise move Monday by government prosecutor Larry Leavitt.

Leavitt said that there was not enough evidence to convict Pandolfo, and that a government witness who testified against Blitzstein had perjured herself.

A motion to dismiss charges against the remaining 10 was denied.

''I'm going to go back home,'' said Pandolfo, a former casino executive at the Stardust Hotel. ''I have to put the pieces back together.''

Later Monday, one of the government's important witnesses, Salvatore Romano, was called to testify in the case against Anthony Spilotro and the nine remaining defendants. The government accuses Spilotro of being the Las Vegas overseer for Chicago crime figures.

Romano, 51, told the court his primary source of income was burglary. He said he moved to Las Vegas in 1980, after becoming a FBI informant in exchange for leniency on some burglary charges in Tucson, Ariz., where he lived for the previous two years.

''I knew I was being squeezed, that they (FBI agents) were wanting me to do things for them,'' Romano testified.

He said he moved to Las Vegas following a phone conversation with Peter Basile, one of the defendants.

''He told me there was a lot of work to be done, a lot of burglaries,'' Romano said.

Romano testified at length about efforts to burglarize a radio shop and the theft of paintings from one of Basile's neighbors.

He acted as an informant for the government until July 4, 1981, when police arrested members of the ''Hole-in-the-Wall Gang'' during a jewelry store burglary that Romano helped set up. Most of the defendants are alleged to be part of the gang, which prosecutors say carried out burglaries and robberies in 1980 and 1981, punching holes in roofs and walls to get into homes and businesses.

Romano is under the government's Witness Protection Program. He said he is being paid about $1,500 a month by the FBI as an informant.