The number of microglial cells in the supraventricular part of the corpus callosum stained with Mac-1 antibody (against CR3 antigens) and the intensity of staining were studied in both the homozygous athymic nude mouse (nu/nu) and normal BALB/c mouse (+/+). For quantitative analysis, the mean microglial cell counts (expressed in terms of packing density) from 40 microns thick immunostained sections were obtained and tested by analysis of variance. The Mac-1 positive cells in neonatal nude mice were slightly less intensely stained than those of their normal littermates. Such was not noticeable in the 13-week- and 1-year-old animals. The mean number of immunopositive microglial cells per 0.0324 mm2 was significantly less in the 5-day-old (P < 0.001) and 13-week-old (P < 0.05) nude mice when compared to normal mice of corresponding ages. The difference was insignificant in the 1-year-old nude and normal mice. The distribution of Mac-1 labelled microglia in different areas of the brain of the postnatal nude and normal mouse was also examined. In the brain areas examined, e.g., the olfactory bulb, cerebral and cerebellar cortex, the number of microglia in the nude mouse was considerably reduced. The study of lectin labelled sections also showed a much smaller number of labelled microglial cells in the athymic mouse. This was especially obvious in the 5-day-old nude mouse when compared to the normal BALB/c mouse (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)