Effective therapy of infections due to facultative intracellular micro-organisms, which persist after ingestion by mononuclear phagocytic cells, requires the use of antibiotics with the ability to inactivate these intra-phagocytic bacteria. Since entry of antibiotics into mononuclear phagocytes is a pre-requisite for activity against such intracellular organisms, we have determined the uptake of 11 radiolabelled antibiotics by human peripheral blood monocytes. beta-Lactams (penicillin G, cefamandole and cefotaxime), gentamicin, and metronidazole had a limited ability to enter monocytes, achieving cellular concentrations which were equal to or less than extracellular levels (C/E less than or equal to 1). Imipenem, a novel beta-lactam antibiotic, rapidly bound to monocytes, but cell-associated drug progressively declined during further incubation. Chloramphenicol, a lipid-soluble drug, and trimethoprim were concentrated three-fold by human monocytes. In comparison with the other antibiotics, roxithromycin (C/E = 14), clindamycin (C/E = 6 to 7) and erythromycin propionate (C/E = 4 to 5) were markedly concentrated by human monocytes. In contrast to our findings in other phagocytes, there was no evidence that active membrane transport was involved in monocyte uptake of clindamycin, erythromycin and roxithromycin. We have demonstrated that several antibiotics are highly concentrated within human monocytes, and it will be important to evaluate the effects of this antibiotic uptake on various monocyte functions, including intra-phagocytic antibacterial activity.