Transferrin and somatomedin C receptors (TFr and SMCr) were studied in human ovaries by immunostaining using monoclonal antisera. The oocyte of evoluting follicles was intensely positive for both types of receptors, but this positivity decreases in large follicles. An evident positivity for both TFr and SMCr was shown in granulosa cells of evoluting follicles. However, not all of these cells were equally immunoreactive. An intense positivity was present in the developing thecal cells of early cavitary follicles as well as in thecal cells of follicles of medium and large size (greater than 6 mm) and in some cells of involuting follicles in initial atresia. These results seem to demonstrate that TFr and SMCr are present in different cellular components of the human developing and early involuting follicles.