The ex vivo effects of macrophage colony-stimulating factor (M-CSF) on antifungal and antibacterial activities of human elutriated monocytes were studied. Cells were isolated prior to the initiation of therapy, on day 3 and at week 7, in six patients with an advanced malignancy receiving M-CSF in a phase I study. Superoxide anion production by monocytes in response to N-formyl methionyl leucyl phenylalanine was enhanced at day 3 of therapy (P = 0.011). In addition, at day 3, fungicidal activity against blastoconidia of Candida albicans was enhanced by M-CSF treatment (P = 0.026), whereas antifungal activity against hyphae of Aspergillus fumigatus was not significantly changed. Bactericidal activity against Staphylococcus aureus was increased at day 3 (P = 0.004). By Northern blot analysis, M-CSF does not upregulate the expression of components of the NADPH-oxidase, the multicomponent enzyme system responsible for generation of superoxide radicals by monocytes. Instead, the predominant effect of M-CSF on circulating monocytes is probably a post-transcriptional effect. In conclusion, these findings suggest that administration of M-CSF to patients may enhance microbicidal activities and thus may provide a useful adjunct to conventional antimicrobial therapy.