There are controversial reports in the literature concerning the effects of opioids on superoxide (O2-) formation in phagocytes, these agents being either inhibitory or stimulatory. We re-examined this issue and compared the effects of the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe), phorbol myristate acetate (PMA), ATP, platelet activating factor (PAF), cytochalasin B (CB) and prostaglandin E1 (PGE1) with those of various opioids on O2- formation in human neutrophils and HL-60 leukemic cells under defined experimental conditions. In the presence of CB, fMet-Leu-Phe and PAF concentration-dependently activated O2- formation in neutrophils with EC50 values of 20 nM and 100 nM, respectively. In the absence of CB, fMet-Leu-Phe and PAF were much less effective. PAF synergistically enhanced O2- formation induced by fMet-Leu-Phe. ATP at a concentration of 100 microM and the opioids, methionine enkephalin, beta-endorphin, dynorphin, [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin, [D-Ala2-D-Leu5]-enkephalin and morphine at concentrations between 10 pM to 1 microM did not activate O2- formation. ATP but not beta-endorphin potentiated fMet-Leu-Phe-induced O2- formation. O2- formation induced by a maximally stimulatory concentration of PMA (100 ng/ml) was enhanced by fMet-Leu-Phe but was unaffected by methionine enkephalin or PGE1. PMA at a non-stimulatory concentration (2 ng/ml) potentiated the effect of fMet-Leu-Phe but did not induce responsiveness to PAF, ATP or beta-endorphin. PGE1 strongly inhibited fMet-Leu-Phe-induced O2- formation, whereas morphine, methionine enkephalin and the opioid antagonist, naloxone, were without effect.(ABSTRACT TRUNCATED AT 250 WORDS)