The macular mutant mouse is a murine model of the Menkes' kinky hair disease, characterized by a copper deficiency in serum. The immune response of its hemizygote (ml/y) was examined, herein. Ml/y mice which were not treated with Cu were atrophy of lymphoid tissues on day 14. However, kidney, brain, heart and lung weights were higher in ml/y mice without Cu treatment than in normal (+/y) mice. When compared to cells from +/y mice, spleen cells from ml/y mice exhibited similar proliferation-curves stimulated by Con A or LPS. Lymph node cells from ml/y mice showed a significantly decreased mixed lymphocyte reaction (MLR) response when stimulated by spleen cells from Balb/c mice. Spleen cells from ml/y mice demonstrated similar stimulation against lymph node cells from Balb/c mice. Antibody production against sheep red blood cells (SRBC) in vivo, a T cell dependent response, was suppressed in ml/y mice. By contrast, the antibody production against dinitrophenyl-ficoll, a T cell independent response was similar in +/y and ml/y. The antibody production against SRBC in vitro was also suppressed in ml/y mice. However, when the T cell-enriched fraction of ml/y mouse spleen cells was replaced by the T cell-enriched fraction of +/y mouse spleen cells, the antibody production against SRBC was recovered. The percentage of Ly-5-positive cells (B cell) from ml/y mice was greater than those from +/y mice. The percentage of Thy-1.2-positive cells (T cell) was decreased, and the decrease was most prominent with the L3/T4-positive T cell (helper T cell) subset.