Gentamicin, trimethroprim, cephalothin, colistin, erythromycin, oxytetracycline and chloramphenicol were studied in a phagocytic system. A radiolabelled strain of E. coli was used as test bacterium and human polymorphonuclear cells were used as phagocytes. Except for trimethoprim and cephalothin, there was a tendency towards depression of the process of phagocytosis in the presence of high concentrations of the various antibiotics. Colistin in a high concentration (83 micrograms/ml) exerted the most significant effect.