The oral cephalosporins cefaclor, cefetamet, Ro 40-6890 and cefpodoxime were studied with regard to their effects to modulate the chemiluminescence response, the rate of phagocytosis, bactericidal action and leukotriene formation from human granulocytes, the release of histamine from human basophils and the induction and release of tumor necrosis factor (TNF) and interleukin-6 (IL-6) from a preparation of human lymphocytes, monocytes and basophils. The cephalosporins increased the histamine release induced by Escherichia coli and Staphylococcus aureus whereas the synthesis of leukotrienes from human neutrophils was decreased. The studies with regard to adherence and phagocytosis showed a significant increase in phagocytosis for E. coli. The bactericidal activity of human granulocytes was also enhanced. Furthermore, the studied cephalosporins decreased the release of IL-6 and TNF.