Research into the mechanisms by which apoptotic cells are phagocytosed has grown considerably over recent years, together with a growing appreciation of the importance of clearance of redundant cells for tissue homeostasis. However, studies addressing the efficacy of phagocytosis have been rare. The few studies reported to date were either attempts to determine apoptotic cell clearance from the circulation or were focused on clearance in inflammation. We now describe an in vitro assay which permits the quantitative measurement of phagocytosis of apoptotic cells by murine resident peritoneal macrophages. The apoptotic cells used in the assay were murine thymocytes incubated with dexamethasone for only 3 h. Most apoptotic thymocytes were annexin V positive and propidium iodide negative and therefore still in the earlier stages of apoptosis. The assay was completed 7 h after the isolation of both macrophages and thymocytes, while macrophage culture time was only 4 h. Because of this short-term culture it is likely that the resident peritoneal macrophages largely maintained their in vivo phenotype. Using BALB/c macrophages and thymocytes, the maximal in vitro phagocytosis exceeded five thymocytes per macrophage in 1 h and two of these thymocytes were taken up within 10 min. Therefore, in vitro phagocytosis by resident peritoneal macrophages was rapid and of high capacity, as it is postulated to be in vivo. Under selected conditions, the mean uptake was 4.45+/-0.70 (mean +/- SD, n = 31) thymocytes per macrophage in 1 h. The inter-assay coefficient of variation, also representing the biological variability, was found to be 15.7%. The average intra-assay coefficient of variation was 13.6%. This assay permits comparisons of phagocytic efficacy between different strains of mice in vitro. In addition, a method of preparation is described which allows long-term storage of experimental results. Finally, our data suggests that internalization, but not binding of apoptotic cells to short-term cultured resident peritoneal macrophages, is critically dependent on the presence of serum. This allows separate analysis of binding and internalization of apoptotic cells with the assay, without the necessity to use agents blocking internalization.