The relative rate of red blood cell (RBC) in vitro phagocytosis by monocytes was measured photometrically by the diaminobenzidinehemoglobin reaction. Our results demonstrate, that the in vitro monocyte-macrophage assay (MMA) allows a specific and sensitive estimation of low RBC-phagocytosis rates. Therefore, the MMA is a useful tool for further characterisation of antibody-monocyte interaction and is of clinical importance in blood group antibody analysis.