Mastomys natalensis infected with the filarial parasite Litomosoides carinii show anaemia and leukopenia. Alterations start with the onset of microfilaraemia. Anaemia is temporally macrocytic (up to 80 days after infection), subsequently normocytic and hypochromic, accompanied by reticulocytosis. Increased intravascular haemolysis (i) and functional disorders of the haemopoetic system (ii) are involved in the pathogenesis. i: Circulating erythrocytes showed increased osmofragility. Hypoglycaemia demonstrated in parasitaemic animals may be one reason. ii: Histological and electron microscopical investigations of the bone marrow revealed markedly enhanced haemopoiesis in infected animals. However, a high proportion of cells was found pathologically altered already beginning in the late prepatency and increasing in the further course of infection. Thus, dyshaemopoiesis may result in the production of morphologically and functionally aberrant cells which are rapidly eliminated by the MPS which is highly activated in L. carinii infected M. natalensis.