Summary MuBl, a complement component of the mouse, occurs in higher concentration in male than in female animals. In the gonadectomized male, the level of MuBl is identical with that of the female, but can be restored to its original value and even increased above its normal concentration by administration of testosterone propionate. MuBl concentration of the female can also be increased by testosterone. Administration of medroxy progesterone acetate leads to a less pronounced increase in concentration. This is found in sham operated females, gonadectomized males and females, but not in sham operated males. Oestradiol monobenzoate slightly augments MuBl levels of gonadectomized female and male animals and of sham operated females. It stabilizes the MuBl concentration of sham operated males, thus causing a relatively lower concentration of MuBl in sham operated, oestrogen-treated males, than in sham operated uninjected males of identical age.