Phagocytic activity of peritoneal exudate cells in mice was stimulated by injection of 0,1 ml mineral oil or varying concentrations of lysolecithin. Optimal lysolecithin concentration was found to be 2 to 5 µg per mouse. The phagocytic activity of peritoneal exudate cells from mice was determined by spectral analysis of dioxane extracts of cells after incubation in the presence of polystyrene latex particles. Maximum uptake of latex particles occurred after an incubation period of 1 h. Maximum phagocytic activity was observed in cells harvested 3 days after stimulation with mineral oil and approximately 4 days after stimulation with lysolecithin. Electron microscopy of these cells revealed a similar pattern of phagocytic activity. Latex particles are inert and not metabolized by the cells and their uptake was therefore compared with the phagocytosis of bluetongue virus particles. Electron microscopic studies of the uptake of bluetongue virus showed that although adsorption of the virus on the surface membrane occurred practically no phagocytosis was observed 1 day after stimulation with lysolecithin. Maximum phagocytosis of virus particles occurred 3 to 4 days after stimulation with lysolecithin.