Within 1–4 days after kidney transplantation mononuclear cell infiltration developed in rabbit homotransplants. The early cells aggregated around interlobular vessels and gradually infiltrated the cortex during the next few days, spreading predominantly through inter‐tubular connective tissue. Swelling of endothelial cells lining medium sized vessels was noted and in transplants surviving for more than 10 days these vessels showed intimal proliferation. Glomerular changes were not prominent. A few immunoglobulin‐containing cells could be detected by fluorescence microscopy in most of the transplants at the time of rejection. The morphological changes are compared with observations made by others, and differences in the results of immunofluorescence studies are pointed out. It is concluded that the results of immunohistological investigations are not comparable as long as no standardization of immunofluorescence reagents and methods e