To clarify the role of local production (exohepatic) of complement on primary host defense mechanisms against microbial infections in the host who decreased the amount of complement in serum, the kinetics of the complement production by the complement-producing cells in exohepatic tissue was examined by measuring the amount of C1q, subcomponent of the first complement component, in cultured supernatant of the monolayer of peritoneal macrophages (PM phi) collected from 5, 15, 35, and 48 weeks-old female mice of NZB/W F1 (B/W F1). The C1q production of PM phi in B/W F1 mice showed remarkable decrease at 15 weeks-old. After that, however, the C1q producibility of PM phi recovered gradually and, at mice 48 weeks old, the complement produced finally exceeded the amount observed in mice 5 weeks-old, contrariwise the C1q values in serum were significantly lowering in the same aged mice. The increased production of C1q of PM phi was observed in both 35 and 48 weeks-old mice, of which the value corresponded with increase of anti-nuclear antibody titer in serum and of the amount of protein in urine. On female mice of ddY (control), the amount of C1q production of PM phi in 5 weeks-old mice was two-fold higher than that of 5 weeks-old B/WF1 mice. But at 15 weeks, the production showed a 1/2 decrease in that of mice 5 weeks-old and the decreased values were kept to 35 weeks-old.(ABSTRACT TRUNCATED AT 250 WORDS)