Normal, thioglycollate-stimulated and BCG-activated mouse peritoneal macrophages were cultivated in vitro with the conditioned medium of mouse L-929 cells. The thioglycollate- and BCG-macrophages rapidly proliferated, whereas normal macrophages grew more slowly. A clear morphological difference between the three types of macrophages in the culture was observed. Glucocorticoids inhibited the growth of the macrophages at pharmacological concentrations. Other steroids, progesterone, diethylstilbestrol and testosterone in that order, had a far lower growth-inhibiting effect. Macrophages cultured with 10(-6) M dexamethasone had a reduced antimicrobial effect on Candida parapsilosis compared with that of the untreated cells. Choleragen had the same effect on the macrophages as glucocorticoids. The toxin inhibited growth at a concentration as low as 10 pg/ml and cells treated with 1 ng of choleragen per ml had decreased antifungal activity. Similarly, Escherichia coli lipopolysaccharide at 10 ng/ml inhibited the growth of thioglycollate-macrophages. However, macrophages incubated with the lipopolysaccharide had enhanced anticandida activity. Thus, the immunosuppressors glucocorticoid and choleragen inhibited both the increase in the number of macrophages and the microbicidal activity of the phagocytes. Lipopolysaccharide, an immunostimulant, stimulated macrophage activity, but was toxic for cell growth.