Peritoneal macrophages from mice injected 4 days previously with Brewer's thioglycollate medium have a pinocytic rate, in culture, of 190 ng horseradish peroxidase (HRP)/100 mug cell protein/h, compared to the rate of resident peritoneal cells of 53 ng HRP/100 mug cell protein/h. Mice injected with endotoxin or with only certain of the components of the Brewer's medium show an intermediate level of stimulation. The rate of unstimulated, endotoxin-stimulated, or thioglycollate-stimulated cells shows little change over several days in culture. The pinocytic rate of thioglycollate-stimulated cells can, however, be further increased by exposure of concanavalin A. Although cells may show transient increases in their pinocytic rate in many situations, a sustained increase in pinocytic rate is a sign of the "activated" state of macrophages.