A former Prince George's County police informant testified today that detectives asked him to get a criminal suspect "involved" in a 7-Eleven store robbery in 1967 so that police waiting at the store could catch him.

The informant, now a prison inmate using a pseudonym to hide his identity, made the assertion under questioning by attorneys for plaintiffs in a $9 million civil rights trial against the police in federal court here, but he appeared to contradict himself later under cross-examination by defense attorneys.

"Is it your testimony now," asked defense attorney James P. Salmon, " . . . that the police did not tell you to set one up recruit a participant for a staged robbery , other than if you knew someone that was going to do one, to get in on it and do it with them?"

"Yes sir," answered the ex-informant, using the pseudonym "Jack Chaney" to protect himself from possible reprisals by fellow prisoners.

Appearing nervous and hestitant throughout his testimony, "Chaney" also acknowledged telling both state and county police investigators in preparation for the current trial that police never asked him to recruit participants for staged robberies--the central accusation of plaintiffs in the case against the police.

"Chaney," who has a conviction record for crimes ranging from armed robbery and auto theft to possession of the illicit drug PCP, was brought to court to testify about one of five holdups and burglaries in 1967 allegedly staged by a so-called "death squad" of Prince George's County police detectives. In the five incidents, police shot and killed two men, wounded a third and arrested several others. Families of the two slain men, plus two of the arrested suspects sued three police officials, including current Deputy Chief Joseph D. Vasco, for $9 million, contending the allegedly staged holdups and burglaries violated due process rights under the 14th Amendment.

In the "Chaney" incident--a robbery of a 7-Eleven store in Cheverly on Nov. 24, 1967--police waiting at the store shot and wounded suspect Pedro Gonzales, who was armed with a .22-caliber revolver. "Chaney," who was also present, was arrested and later released.

Police have denied staging the holdup, contending "Chaney" reported to them that Gonzales planned the action and that police, as a matter of routine, then staked out the store. "Chaney" said he had talked to both Vasco and former detective James Fitzpatrick about the holdup. Fitzpatrick is now a captain in charge of the police department's major crimes division.