Functional assays intended to reflect the haemolysis in vivo have been performed with peripheral monocytes, occasionally only with peritoneal macrophages or monocyte-like cell lines. Although the splenic macrophages presumably play the major role in immune red cell destruction, the functional assays have not been done with these cells because of their relative inaccessibility. In this study, the activity of peripheral monocytes and splenic macrophages was compared using the monocyte monolayer assay (MMA), and no substantial differences were found.