Sex hormones control variations in the serum levels of C4-binding protein (C4-bp) in mice. The levels of C4-bp in castrated males are decreased as compared to the sham-operated controls. Castrated females injected with testosterone have higher levels of C4-bp than those injected with the vehicle alone. Effective “masculinization” of castrated females, with respect to C4-bp levels, was achieved by periodic inoculations of testosterone, and “feminization” of males was obtained by orchidectomy. In this regard, C4-bp behaves like other C components (C4, C5, C6) whose serum levels are strongly influenced by sex hormones.