The curious French crime thriller "La Balance," which opened yesterday at the Outer Circle, is a divertingly vicious reprise of the hard-boiled tradition, dusted off by an American expatriate filmmaker, Bob Swaim.

The title is said to derive from an underworld euphemism for informers. In Swaim's movie the officers of the 13th Territorial Brigade, which monitors the Belleville district of Paris, are obliged to compensate for the murder of a key informer, Paulo, shot down by agents of a criminal big-shot named Massina (Maurice Ronet, radiating sinister composure). Massina also circulates rumors alleging that the cops did the killing.

It's a challenge that can't go unanswered. The resident Machiavelli of the 13th Brigade, newcomer Richard Berry as the insinuating Inspector Palouzi, engineers to plant an undercover agent in Massina's gang. His patsy is a paroled thief named De'de' (Philippe Leotard), who lives with a prostitute named Nicole (Nathalie Baye). When the couple resists, Palouzi puts on the screws with double effectiveness, pressuring both De'de' and Nicole into acting as unwilling and now mutually suspicious accomplices. The inspector succeeds in setting up Massina, but his scheme also backfires.

"La Balance" unfolds with fairly dependable tension and excitement, but the lurid milieu and characters never concentrate our attention. Morally there's not much to choose among Palouzi, De'de' and Nicole, even in the cleverest of melodramatic circumstances, and the film barely begins to explore this three-cornered, corruption-stained ambiguity. On the contrary, it's content to make do with thumbnail characterizations.

Berry is the most striking performer. He projects the most alarmingly slimy presence since Ben Gazzara made his film debut in "The Strange One." He's also oddly dwarfed and upstaged in a few scenes by a tall, narrow-faced, bluntly amusing actress named Anne-Claude Salimo, cast as a streetwalker who seems to glare down at Palouzi from a very intimidating height.

I was also intrigued by a curious fashion eccentricity: Is Berry breaking in some inexplicable style by appearing every so often with shirt cuffs rolled down and yet unbuttoned? Customarily, this kind of disarray suggests absentmindedness, but in Palouzi it must betoken a deliberate touch of looseness, perhaps emblematic of his off-the-cuff virtuosity as a detective. Anyway, it's a new one on me--perhaps the only new one that "La Balance" really leaves in its vividly trashy, trifling wake. LA BALANCE

Directed by Bob Swaim; produced by Georges Dancigers and Alexandre Mnouchkine; director of photography, Bernard Zitzermann; edited by Francoise Javet; music by Roland Bocquet. Presented by Spectrafilm. Running Time: 102 minutes. Rated: R. THE CAST Nicole . . . . Nathalie Baye Dede . . . . Philippe Leotard Palouzi . . . . Richard Berry Tintin . . . . Christophe Malavoy Le Belge . . . . Jean-Paul Connart