1. In this paper acetylsalicylic acid (aspirin), an anti inflammatory drug, was studied in vitro at doses of 50, 100, 200 and 500 mg/l to see its effects on adherence, chemotaxis, spontaneous mobility, phagocytosis, candidicide power, nitrobule tetrazolium (NBT) reduction as well as the incorporation and metabolism of arachidonic acid in human polymorphonuclear neutrophils (PMNns). 2. Aspirin significantly stimulated neutrophil adherence to nylon fiber at all the doses used, with a correlation between the doses used and the adherence indices found. 3. At the therapeutic dose (100 mg/l) aspirin brings about a significant increase of chemotaxis, but reduces this property at the highest dose (500 mg/l). On the other hand, spontaneous mobility is not altered except with the 500 mg/l dose of aspirin which produces a significative decrease. 4. The ingestion of Cándida albicans by PMNns is significant at the therapeutic dose; the candidicide power is not modified with any of the doses used with 100 mg/l of aspirin nitroblue tetrazolium (NBT) reduction is significantly increased. 5. No changes are observed in the incorporation of arachidonic acid or in the release of its metabolites.