51Cr studies showed that after complete eradication of malarial parasites erythrocyte life-span is reduced for 4-5 weeks and that the reduction is associated with the presence of complement-containing immune complexes on the red-cell surface. The rate of erythrophagocytosis in the spleen is increased during this period. 59Fe studies of erythropoiesis indicated a mild degree of marrow depression. A very important factor causing anaemia in malaria appears to be a complement-mediated immune process.