Picture this: Harry Langdon, well past his silent-comic stardom, cast as a Communist streetsweeper, half-singing his denunciations of "Those idle rich shirkers/Who live off us workers." Hallelujah, I'm a Bum is far from great, but it has to be among the most unusual films ever to come out of Hollywood. The swatches of Lorenz Hart's "rhythmic dialogue" resemble earlier experiments by René Clair, Rouben Mamoulian and Ernst Lubitsch, but none of them took their poets from Central Park tramps and thus none made a film half so odd. To judge from its box-office reception, the Depression-era public found little charm even in Al Jolson's engaging singing of "I find great enjoyment/From unemployment" to his black pal Edgar Connor. As set to Richard Rodgers' impressionistic music, Jolson can crank out a routine from singing a postcard: "Dear June: I got to Cleveland OK...." His performance as the bums' leader is much more appealing than in his breakthrough early-sound films, even if his exuberance, as always, looks a bit forced. The storyline is a mess, involving complications with a lost billfold and Jolson's rivalry with the mayor of New York (Frank Morgan, subsequently the Wizard of Oz) over an amnesiac woman in slinky gowns. ("She dresses like a capitalist," scorns Langdon. "That's OK; some of my best friends are capitalists," responds Jolson.) Lewis Milestone directs with a sure touch-as if he at least knew what was up-and the film is never less than surprising. The Library of Congress print of the complete version of this often-cut film is among the most beautiful of those shown this week. The program begins with a test reel made before principal photography, in which the leads try out their costumes. Scott Simmon