The macular mutant mouse is a murine model of Menkes' kinky hair disease, characterized by a serum copper and ceruloplasmin deficiency. The hemizygote (ml/y) had a decreased antibody response to sheep red blood cells (SRBC). 1) In vivo; the administration of ceruloplasmin recovered antibody production in ml/y mice, but, had no effect on that of normal mice (+/y). The administration of heat treated ceruloplasmin did not increase antibody production in ml/y mice, whereas the administration of various concentrations of copper suppressed the anti-SRBC plaque-forming response in both ml/y and +/y mice. 2) in vitro; ceruloplasmin had no effect upon antibody production against SRBC in both ml/y and +/y mouse spleen cells. In contrast to ceruloplasmin, 1 microgram/ml of copper inhibited the antibody production in spleen cells from both +/y and ml/y mice as well as in vivo. These findings suggested that the decreased antibody production in ml/y mice was considered to be due the decreased ceruloplasmin.