Megakaryocyte growth and development factor (MGDF) administered intraperitoneally (IP) to mice causes a dose-dependent thrombocytosis accompanied by a decrease in mean platelet volume. MGDF increases the number of megakaryocytes in the bone marrow and spleen. MGDF does not affect the circulating number of leukocytes. Carboplatin, a chemotherapeutic agent that causes thrombocytopenia in humans, administered to mice as a single IP injection at a nonlethal dose causes a significant, but reversible thrombocytopenia. The carboplatin- induced thrombocytopenia is accompanied by an increase in circulating endogenous MGDF that precedes the return of circulating platelets to a normal level. MGDF mRNA is constitutively present in the liver. After carboplatin treatment, hepatic MGDF mRNA does not increase in concordance with circulating MGDF. Circulating soluble MGDF receptor levels (c-mpl) do not change significantly during the course of carboplatin-induced thrombocytopenia. MGDF injected IP once daily beginning 1 day after injection of carboplatin reverses carboplatin- induced thrombocytopenia in a dose-dependent fashion. The normalization of circulating platelet numbers in carboplatin plus MGDF-treated mice is accompanied by a normalization of megakaryocyte numbers in the bone marrow. In conclusion, MGDF, by increasing the number of marrow megakaryocytes and circulating platelets is an effective therapy for carboplatin-induced thrombocytopenia in mice.