A spectrometric assay for assessing erythrophagocytosis by mononuclear phagocytes is described. It is based on the haemoglobin-catalyzed conversion of a benzidine derivative into a coloured product in the presence of H2O2 (pseudoperoxidase activity). The assay is set up in microtitre plates, and following an uptake phase and removal of non-ingested erythrocytes, pseudoperoxidase activity is measured in detergent lysates of phagocytes, using an ELISA reader photometer. Various detergents and substrates were evaluated. SDS was found to be the most suitable detergent. Diaminobenzidine (in PBS, pH 7.4) was the substrate of choice for enumerating ingested erythrocytes in a range from 10(4) to 5-8X10(5) sheep erythrocytes. Ortho-tolidine (in acetate buffer, pH 5.5) could be used in a range from 2X10(3) to 2X10(5) sheep erythrocytes. The results obtained with human peripheral blood monocytes or monocyte-derived macrophages and IgG-sensitized sheep erythrocytes correlated well with those obtained using 51Cr-labelled, IgG-sensitized erythrocytes.