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Although phagocytosis of micro-organisms by granulocytes is one of the most important defence mechanisms against infection, little is known about the kinetics of this process. The present study showed that the rate of ingestion of Staphylococcus aureus and Escherichia coli depends on the concentrations of the granulocytes and bacteria. Phagocytesis of bacteria at a bacteria-to-cell ratio in the range between 100:1 and 1:10 showed an exponential course during the first 30 rain. At a bacteria-to-cell ratio of 1:1, application of a correction for the outgrowth of extracellular bacteria gave an exponential course of ingestion over the first 90-min period. Since it was found that the phagocytosis of bacteria by granulocytes at various bacteria-to-cell ratios can be described with Michaelis-Menten kinetics, we studied the kinetics of phagocytosis on the basis of the initial rate for the first 30-min period. The rate of phagocytosis and the maximal degree of ingestion of bacteria by granulocytes proved to be related to the concentration of serum used in the assay. The minimal serum concentration required for maximal ingestion was 2·5% for Staphylococcus oureus and 5% for Escherichia coli. When bacteria were preopsonized, the duration of pre-opsonization proved to be limiting for the rate of phagocytosis in dependence on the serum concentration. The effect of temperature on the phagocytosis of micro-organisms proved to be two-fold. First, at temperatures between 4 and 33′ a decrease in the functioning of the cells leads to a decrease in the rate of phagocytosis. Above 42°, the temperature affects mainly the opsonization of the micro-organisms and has only a slight influence on the ingestion process. From the data obtained in this study, maximal rates of 6·3 × 106 Staphylococcus aureus/5 × 106 granuloeytes/min and of 7·1 × 106 Eseherichia coli/5 × 106 granulocytes/min were calculated for phagocytosis at a bacteria-to-cell ratio of 100:1 at 37°, i.e. on average about one bacterium per granulocyte per min. The maximum calculated number of bacteria ingested by one granulocyte lies between 40 and 50. [ABSTRACT FROM AUTHOR]