Testicular biopsy specimens from fifteen normal adult men were analysed histologically, using Clermont's classification for the germ cell types. After corrections were made for differences in nuclear size and life-span, spermatid/primary spermatocyte ratios ranging from 0·87 to 3·82 (average 2·58) were found. This represents a marked reduction from the theoretical ratio of 4·0 which would be expected. The significance of this attrition rate, or the point at which loss occurs, is not known.