We constructed transgenic mice that carry the cDNA of human interleukin-2 (IL-2) under the control of the H-2Kd promoter. The IL-2 transgenic mice expressed human IL-2 mRNA in the thymus, spleen, bone marrow, lung, muscle and skin. Human IL-2 protein was also detected in their sera. The IL-2 transgenic mice suffered from alopecia and pneumonia, but no typical autoimmune reactions seemed to be involved in these lesions. In the epidermis of the IL-2 transgenic mice there was an increase in Thy-1+ dendritic epidermal cells (DEC), which might be involved in the skin lesion. Immune responses of their spleen cells against antigens were significantly impaired whereas their spleen cells responded well to polyclonal lymphocyte activators.