Peritoneal rat macrophages and rabbit polymorphonuclear leukocytes were obtained after thioglycollate or glycogen stimulation. Optimal conditions for phagocytosis were determined by a recently developed quantitative fluorimetric assay. We studied by serial SEM micrographs macrophages and polymorphonuclear leukocytes incubated either in medium or in the presence of different types of phagocytic particles. We compared the morphological aspects of adsorption and phagocytic processes for opsonized microorganisms (Micrococcus lysodeikticus and Candida albicans) with these for inert beads of iron and metallic mercury. Without phagocytosis, in the presence of fresh homologous serum, we observed a progressive development of microvilli or lamellipodia in ruffles and by the end, hypertrophied ruffles appeared at one pole of the cell. We noted extremely well developed ruffles during phagocytosis of opsonized microorganisms. These were practically absent on the macrophages incubated with inert particles. The mean number of adsorbed particles is more elevated in the case of iron and metallic mercury beads than for microorganisms. The rate of ingestion of inert particles was considerably higher than for microorganisms even when they were opsonized. In conclusion, at all stages of the phagocytic process, we observe different morphological features of the macrophages depending on the nature of the phagocytosed particles.