The origin of glomerular crescents and diffuse hypercellularity was investigated in rabbits receiving a single intravenous injection of nephrotoxic serum. Experiments to demonstrate local glomerular proliferation included mitotic counting at intervals after nephrotoxic serum and autoradiography of kidneys 1 hour after pulse-tritiated thymidine labeling. There was an increase in local endocapillary proliferation between days 4 and 6 when severe diffuse hypercellularity developed. From days 8 to 13 during crescent formation there was considerable local epithelial proliferation. To assess the role of infiltrating bone marrow cells a new technique of renal transplantation in rabbits was developed. Kidneys from unlabeled donors were transplanted on either day 4 or day 9 into recipients prelabeled with tritiated thymidine, and labeled cells were sought in glomeruli of transplants 1 to 4 days later. Large numbers of labeled cells appeared on days 5 to 6 in areas of endocapillary hypercellularity in glomeruli of animals given transplants on day 4 indicating a significant influx of bone marrow cells. In contrast, in kidneys transplanted on day 9, few labeled cells were present in crescents on days 10 to 11. Autoradiographic labeling of mononuclear phagocytes in carrageenan granulomas and interstitial renal infiltrates in recipients were used as positive controls of bone marrow cell labeling. We conclude that in the early phase of diffuse hypercellularity there is considerable blood monocyte infiltration, whereas crescents which develop later are principally the result of glomerular epithelial cell proliferation.