The effect of antibiotics on phagocytosis and killing of Pseudomonas aeruginosa by rabbit polymorphonuclear leukocytes was studied. Carbenicillin and sulbenicillin, when added to an incubation medium at a concentration as low as 1/16 MIC, increased phagocytosis and killing of P. aeruginosa by PMN. Meanwhile, gentamicin and 3’,4’-dideoxykanamycin B gave no influence on the PMN activity, and polymyxin B and colistin enhanced the activity only at MIC. The PMN activity was not facilitated even when the cells of P. aeruginosa had been pretreated with antibiotics. The bactericidal activity of PMN decreased after sonification, but was restored following addition of carbenicil