SUMMARYKidneys were transplanted between inbred strains of rats, and the renal interstitium was examined serially by light, immunofluorescence, and electron microscopy. In addition to observations reported by various authors previously, several observations not previously described or emphasized were made during this study. They include direct cytoplasmic communications between mononuclear cells, prominent replication of mononuclear cells within the allograft, and abundant collagen production within the allograft. No morphological evidence was found during this study to support the view that the mononuclear cells destroy the renal parenchyma by antibody-mediated mechanisms or by contact-mediated cytotoxicity.