SummaryChronic protein‐deficiency in weanling mice caused variable suppression of the Immoral and plaque‐forming cell (PFC) responses to sheep erythrocytes. This was most prominent at high antigen doses and did not increase when mice were maintained on the diets for longer periods. Antibody responses produced by deficient mice were often short‐lived and invoked high levels of IgM. Total PFC counts were depressed slightly more than were circulating antibodies. Antibody responses toBrucella abortuswere slightly decreased by protein‐deficiency at high antigen doses hut were normal or elevated at lower doses, the proportion of IgM produced was increased and the splenomegaly response toB. abortuswas severely depressed. These results suggest that the depression of antibody production by protein‐deficiency is not simply due to an impairment of helper T cell function, hut a reduction in the availability or effectiveness of macrophage and regulatory or suppressor T cells may be important.