The effect of FSH on the development of the testis in the ram lamb was examined in two experiments where lambs were passively immunized against ovine beta-FSH from birth until 100 or 160 d. In both experiments, immunization resulted in a slower testicular growth relative to that of controls. This effect became apparent at around the start of the period of rapid testicular growth (60-70 d of age) and resulted in testicular weights at the end of treatment ranging from 37 to 51% of those of control groups. Within the testis, this was reflected in shorter seminiferous tubules (48-64% of controls) and in lower numbers of Sertoli cells per testis (57-82%). In the rams immunized until 160 d of age, spermatogenesis had begun and immunization against FSH provoked a lower production of germinal cells which was not solely due to the lower number of Sertoli cells but also due to fewer germinal cells being supported by each Sertoli cell. However, the numbers of A0 spermatogonia per testis and the daily production of the A1 spermatogonia were unaffected by immunization, but the production of the B2 spermatogonia and, as a consequence, of leptotene and pachytene spermatocytes and of round spermatids were all markedly lower (43-47% of controls). These effects were not due to any decreases in the secretion of LH or testosterone as seen in the blood levels of these two hormones. These results show that, in the ram lamb, FSH is essential for normal testicular development and for the establishment of a normal population of Sertoli cells. They also confirm that, once spermatogenesis is established, FSH is necessary for a normal production of germinal cells, with one or more of the divisions between the A1 and B2 spermatogonia being sensitive to suppression of FSH.