Copper(II)2(3,5-diisopropylsalicylate)4 [Cu(II)2(3,5-DIPS)4] is a synthetic copper complex with a variety of effects, including radiation recovery, anti-inflammatory, and accelerated wound healing activities. When C57BL/6 mice were injected subcutaneously with 80 mumol/kg Cu(II)2(3,5-DIPS)4 their spleens significantly enlarged. This splenomegally lasted for at least 3 weeks and was accompanied by increased myelopoiesis in the spleen. Bone marrow had no significant change in cellularity or myelopoiesis. The treatment with Cu(II)2(3,5-DIPS)4 had only minor effects on the ability of spleen cells to respond to mitogenic or antigenic stimulation. Cu(II)2(3,5-DIPS)4 did stimulate lymphopoiesis, since it accelerated the recovery of immune reactivity following exposure to 8 Gy whole body irradiation. Copper levels in the spleens and bone marrow of unirradiated mice treated with Cu(II)2(3,5-DIPS)4 were not significantly elevated 24 hr after treatment and copper levels were transiently reduced 7 days after treatment.