A luminol-dependent chemiluminescence assay for the assessment of the phagocytosis of erythrocytes sensitized w i t h anti-D IgG immunoglobulin by mononuclear leukocytes is described. The mononuclear leukocytes were obtained by apheresis enriched by centrifugation through a density gradient and stored in liquid nitrogen before use. The total reaction mixture, consisting of mononuclear leukocytes-luminol-erythrocytes (either anti-D IgG sensitized or unsensitized controls) was 500 PI, light detection was by an LKB 1251 luminometer. Peak luminescence was seen between 35-45 minutes, the reaction being exhausted by 120 minutes. Determination o f the reproducibility of the assay gave intra-and inter-assay coefficients of variation of 5 % and 13 % respectively. We found the chemiluminescent response t o be affected by the number of erythrocytes used in the assay and by the composition of the medium i n which the cells were resuspended, particularly the pH at the initiation of the assay. We also compared the chemiluminescence assay t o a microscopic phagocytic assay and found the results virtually identical. However, the former chemiluminescence assay was much easier t o perform, marginally more sensitive, less laborious and eliminated any possibility of subjective error.