The lactate dehydrogenase mouse mutant Ldh-1c/Ldh-1c is afflicted with a severe hemolytic anemia associated with extreme reticulocytosis (95%) and splenomegaly. Ninety-one percent of the total body colony-forming units--erythroid (CFU-E) have been quantified in the seven- to ten-times enlarged spleens of the mutant mice. Moreover, the splenic fraction of morphologically recognizable erythroid precursors was 134 times normal. From these data it was apparent that the spleen crucially contributes to the maintenance of steady state erythropoiesis in the mutants. On the other hand, an enhanced sequestration of red blood cells in the enlarged spleen may augment the anemia. Splenectomy experiments were performed with LDH mutant and wild type mice in order to investigate the role of the spleen in this particular hemolytic disease. Following splenectomy, the peripheral blood values and the frequency of femoral stem and progenitor cells were determined, and histological investigations were carried out. The life span of the splenectomized mutants was not shortened, in spite of a very low red blood cell count (25% of the untreated mutant value). Compared to the splenic loss only a moderate increase in bone marrow erythropoiesis was observed, such as a 250% increase of CFU-E. It is concluded that the reduction in red blood cell survival due to splenic sequestration in the mutants is of such a magnitude that it counterbalances a significant portion of splenic erythropoiesis.