Injection of approximately 10(6) to 10(9) Escherichia coli into the renal arteries of rabbits resulted in retention of sufficient numbers of organisms in renal vessels to permit study of the mechanism of localization by electron microscopy. After injection of the bacterial suspension, perfusion fixation was used to maintain open vascular contours. Individual organisms were found to adhere to the endothelium of glomerular and intertubular capillaries, and ruthenium red staining demonstrated a close interaction between the largely polysaccharide bacterial microcapsule and the sialoglycoprotein endothelial surface coat. Thus, individual E. coli seem to localize in the rabbit kidney in this model by sticking to the endothelial surfact coat of the renal vessels. After localization, polymorphonuclear leukocytes and monocytes appeared in the capillaries and phagocytosed the bacteria. Phagocytosis of bacteria was evident at 10 minutes and was almost complete at 60 minutes and, although less frequent, were associated with small amounts of fibrin at 60 minutes. This acute inflammation and thrombosis may be mediated by activation of complement through the alternate pathway.