Cambridge police said yesterday they have a definite suspect in the theft of more than $250 from Liberal Union film proceeds, but would not say whether they would make an arrest.

Two detectives spent yesterday questioning members of the organization and others and have narrowed down suspects to one undergraduate.

Liberal Union members requested investigation Wednesday following discovery of a loss of more than $180 in film proceeds from "The 39 Steps," shown Tuesday at the New Lecture Hall.

H.L.U. May Press Charges

Informed of the identity of the student last night, the Liberal Union Executive Board decided to press charges against him unless he returns the money missing from previous shows or agrees to undergo lie-detector and other examinations to clear himself of the previous thefts. The suspect is not a member of the Liberal Union.

Discovery of the theft came after a cross-check of ticket stubs against total income revealed that the organization received no money for $180 worth of tickets.

Yesterday the detectives spent hours talking with several students, and from their discussions are satisfied they know how and by whom the crime was committed.

Because of the proximity of Christmas, and because the student involved is an upperclassman, the detectives implied they would probably be partially satisfied with restitution of the stolen mnoey.

Restitution, however, would not necessarily release the suspect, police said. It might aid more lenient prosecution, but if the detectives feel that anyone is guilty, they will serve a warrant for grand larceny.