Electron microscopical investigations on in vitro and in vivo interactions of normal Staphylococcus aureus cells with rat peritoneal macrophages showed that these bacteria were rapidly endocytosed and digested even in the absence of specific antibodies. In contrast to the parental strains oxacilin-induced and stable variante lacking a cell wall (L-forms) were ingested without subsequent formation of phagolysomes and digestive vacuoles. The intracytoplasmic L-form bodies retained their characteristic ultrastructure, i.e. no visible alterations occurred. Some morphological aspects of the L-forms and their persistence in macrophages 7 days after intraperitoneal administration of L-form to rats, suggest the possibility of their intracellular survival.