Phagocytosis of microcapsules by guinea-pig polymorphonuclear leucocytes (PMNs) was examined particularly considering the method of measurement and the effects of surface negative charges of the microcapsules and plasma proteins. Estimation of phagocytosis by oxygen consumption was found to correlate fairly well with the measured amount of the microcapsules phagocytosed. In the absence of plasma proteins, microcapsules having the same surface negative charge as that of the PMNs underwent less phagocytosis than those having surface negative charges different from those of the PMNs. The presence of plasma proteins was found to suppress phagocytosis.