Brewer thioglycolate-elicited mouse peritoneal macrophages were as active as resident peritoneal macrophages in the phagocytosis of opsonized Staphylococcus epidermidis but were unable to kill ingested microorganisms. This decreased functional activity was restricted to Brewer thioglycolate-elicited macrophages, since peritoneal macrophages elicited with NIH thioglycolate, alone or supplemented with agar and methylene blue, were as active as resident peritoneal macrophages. No effect of agar on the functional activities of macrophages was observed. A defective intracellular killing by peritoneal macrophages due to Brewer thioglycolate was seen only after an intraperitoneal injection with thioglycolate, not after in vitro incubation of resident macrophages with thioglycolate. The results of this study show that, depending on the kind of thioglycolate used, the functional characteristics of elicited macrophages may alter. However, none of the forms of thioglycolate investigated induced the recruitment of activated macrophages.