Existing data concerning the effects of x-irradiation on spermatogenesis in man were analyzed and the results were compared to published data on the mouse. Testicular x-irradiation produced a transient, but substantial, suppression of sperm counts in man, with an ED50 near 11 rad. The length of time to recovery was proportional to the irradiation dose. The ED50 for suppression of type A spermatogonia following radiation exposure in man was similar (9.7 rad), although the response curves for spermatogonia and sperm count were matogonia in the mouse was parallel to that found in man, but with an ED 50 of 30.0 rad. These results suggest that, compared to the mouse, spermatogenesis in man is approximately 3.1 times more sensitive to ionizing irradiation.