A micro-assay has been developed for the separate evaluation of phagocytosis and intracellular killing of Staphylococcus aureus by human monocytes and granulocytes. The minimal number of phagocytes required for the investigation of these functional activities of phagocytic cells has been established by performing phagocytosis and intracellular killing experiments at various cell concentrations, bacteria-to-cell ratios, and volumes. The results of these experiments revealed that phagocytosis can be measured in a reliable way, at bacteria-to-cell ratios of 5:1 and 1:1 (cell concentration 5 x 10(6)/ml), in a suspension of 200 microliters. The rate of intracellular killing by monocytes and granulocytes can also be measured with a total phagocytic suspension of 200 microliters and a cell concentration of 5 x 10(6)/ml. From these results it can be concluded that for an independent determination of the phagocytosis and intracellular killing of micro-organisms, 400 microliters of 5 x 10(6) phagocytes/ml is required, i.e., a total of 2 x 10(6) phagocytes. This number of granulocytes can be obtained from 1-2 ml of blood; for monocytes 4-8 ml of blood is required.