1. Fenclozic acid (2-(4-chlorophenyl)thiazol-4-ylacetic acid; I.C.I. 54,450; "Myalex") is one representative of a new class of compounds with antiinflammatory, analgesic and antipyretic properties as evidenced by its activity in a variety of tests in rats, mice and guinea-pigs.2. In tests of short duration the potency of fenclozic acid is similar to that of phenylbutazone.3. In tests of longer duration fenclozic acid is more potent than phenylbutazone.4. The activity of fenclozic acid is not mediated by stimulation of the adrenals and the compound is devoid of corticosteroid-like activity.