Sexual dimorphism of mouse complement component 5 (C5) was detected by isoelectric focusing of desialated ethylenediamine tetraacetic acid (EDTA)-plasma on agarose gel, followed by immunofixation with anti-mouse C5. Male plasma displayed two C5 bands, basic and acidic, while female plasma had only a basic C5 band. In all mouse strains tested except C5-deficient strains, the identical patterns of the dimorphism were obtained. The basic and acidic C5 were antigenically and hemolytically indistinguishable from each other. In the neonatal mice of both sexes, the basic C5 was observed at the same levels, but the acidic C5 was only slightly detected. Injection of testosterone into mice resulted in an appearance of the acidic C5 in females and its increase in males. By contrast, injection of estradiol decreased the acidic C5. These results indicate that the expression of the acidic C5 is under the control of testosterone.