Dietary copper deficiency decreases the number of splenic CD4 cells and mitogen-induced generation of interleukin-2 activity and DNA synthesis in cultures of splenic mononuclear cells. To determine the reversibility of these defects, Cu-deficient rats were fed a Cu-adequate diet for either 4, 7 or 11 d before preparation of cell cultures. Serum and hepatic concentrations of Cu attained 87 and 75%, respectively, of the control level after 4 d of dietary repletion. In contrast, interleukin-2 activity and [3H]thymidine incorporation in splenic cell cultures treated with T-cell mitogens were significantly greater than in cultures from Cu-deficient rats after 7, but not 4, d of dietary Cu repletion. The number of splenic CD4 cells was also greater after 7 d of dietary supplementation with Cu. Changes in the relative percentage and function of T-helper cells were highly correlated with one another and with hepatic Cu concentration. These observations indicate that an inadequate supply of dietary Cu reversibly suppresses the maturation and function of splenic T-helper cells.