Antisera to human C4 can discriminate circulating Ss protein (C4) levels in mice. Since there has been no information on early complement components (C1, C4, C2) in the renal lesions of B/W mice, we applied the indirect immunofluorescence technique to post-mortem sections of kidney from B/W female mice with advanced renal disease. C4 was present in fifteen of the sixteen specimens, usually in a distribution similar to that of IgG or C3. Specificity was demonstrated by differential absorptions with high-Ss serum from C57BL/6 male mice and low-Ss serum from C3H/HeJ female mice. High-Ss-absorbed antiserum did not stain, while low-Ss-absorbed antibody retained much of its activity. This finding parallels the demonstration of early complement components in lesions of clinical lupus nephritis, and is consistent with classic complement pathway activation in B/W disease.