Investigations on the possible hazards of the use of internally administered radioisotopes in human medicine either as therapeutic or diagnostic agents before or during child bearing age are of late gaining importance. The present investigation has been taken up to screen the effects of sulphur-35 on spermatogonia. CBA pregnant mice were injected (ip) with a dose of 20 ..mu.. Ci of sulphur-35 on 3.5, 10.5 or 15.5 days of gestation. At the similar intervals pregnant mice injected with physiological saline were kept for control data. All the animals were allowed to litter and F/sub 1/ male progeny were killed at maturity at the age of 10 weeks and the testes collected. Sections of both the testes were prepared and stained by PAS-haematoxylin technique and the survival of spermatogonia types A, Int and B and preleptotene spermatocytes was evaluated. There was a significant reduction in all the cell types in the sulphur-35 treated animals. Thus the results indicate the cell-killing effect of radionuclide.