In order to investigate the importance of the transplantation site for the replication of grafted islet cells, we implanted syngeneic mouse pancreatic islets intrasplenically, intraportally and subcapsularly in the kidney. Fourteen days later the alloxan-diabetic mice were killed after an injection of tritiated thymidine, and the graft-bearing organs fixed and processed for autoradiography. The highest labelling indices were recorded for subcapsularly grafted islets, followed by intraportal and intrasplenic islets in that order. In separate experiments some islet-containing kidney sections were immune stained for insulin before the autoradiographic process. The labelling index of the insulin-positive cells was as high as in the entire islet cell population of the sections from the same mice stained with haematoxylin only. This indicates that the B cells of the islets replicate as often as the other islet cell types. The present data also suggest that the renal subcapsular space offers better growth conditions for transplanted islet cells than the liver or spleen.