1. We have investigated the effects produced in vitro by Cefmetazol, Cefoxitin and Imipenem on the chemotaxis, spontaneous mobility, adherence, phagocytosis and candidicide power of human polymorphonuclear neutrophils (PMNs). 2. The three antibiotics tested significantly stimulated the adherence of neutrophils to nylon fibre at doses either equal (50 mg/l) or superior (500 mg/l) to the therapeutic one. 3. Cefmetazol, Cefoxitin and Imipenem bring about a maximum increase of chemotaxis at the therapeutic dose, whereas the spontaneous mobility diminishes with any one of the doses used. 4. The capacity of the PMNs to phagocytize and produce lysis of Candida albicans is increased in Cefmetazol with therapeutic doses. Cefoxitin produced an increased lysis of candidas.