Resident peritoneal macrophages were obtained from untreated mice and were cultured in medium 199 with or without 5% acid-treated fetal bovine serum. Three hours after harvesting, redox compounds--i.e., methylene blue, methyl viologen, or nitro blue tetrazolium--were added to the cultures of adherent cells. After 1 hr, the cells were washed and culturing was continued in the absence of redox compounds. The effects of the redox compounds were tested by assaying for hexose monophosphate (HMP) shunt activity and for plasminogen activator secretion, and the results were compared with the effects induced by phagocytic stimuli. Methylene blue caused a concentration-dependent stimulation of the HMP shunt, whereas methyl viologen and nitro blue tetrazolium were ineffective. Shunt stimulation by methylene blue was followed, after a lag of 2-4 days, by plasminogen activator secretion. The rate of secretion was dependent on the methylene blue concentration used. Methyl viologen and nitro blue tetrazolium were again ineffective, whereas phagocytosis of zymosan or sheep erythrocytes, which stimulates the HMP shunt, induced plasminogen activator secretion at rates similar to those induced by methylene blue. These results add further evidence to our hypothesis that the HMP shunt-dependent metabolic burst is involved in macrophage activation. Because methylene blue mimics the action of zymosan it appears that shunt stimulation by itself initiates the activation process independently of phagocytosis.