Phagocytosis is an important function of polymorpho-nuclear cells and macrophages, closely related to the immune defense of the organism. Phagocytic cells engulf not only bacteria, but also other cells, as well as inert particles, such as latex. One method for evaluation of phagocytosis is based on incubation of phagocytic cells with latex particles in vitro at maximal incubation conditions which require not only careful isolation and purification of the cells, but also applying meticulous techniques for maintaining constant temperature, suitable pH, and incubation media [1]. Even if the incubation is carried out under the best possible conditions, the withdrawal of the cells from their physiological milieu presents a serious trauma which may impair their phagocytic capacity. Hereby, we suggest a simple method which permits in vivo evaluation of the phagocytic function of rat peritoneal macrophages and polymorphonuclear cells under maximal, almost physiological conditions. An amount of 5 ml of a 5% suspension of uniform polystyrene latex particles (0.8 µm in diameter; Difco, Detroit, Mich., USA) was injected into the peritoneal cavity of Wistar rats weighing 330-400 g. The animals were kept in their cages at room temperature for 60 min. To obtain the cells, the peritoneal cavity was ‘flushed’by injection of 60-80 ml of phosphate-buffered saline, followed by gentle shaking and rotation of the animals for 5 min. The intraperitoneal fluid was withdrawn, and the cells were sedimented by centrifugation at 250 g for 10 min. Smears stained by the May-Grünwald-Giemsa method were examined under a t! P