Prenatal exposure to the dye Congo red causes a reduction in the number of germ cells in male and female offspring. In the current investigation, nine other dyes structurally related to Congo red were evaluated for developmental testicular toxicity and the structural component of the dyes responsible for the prenatal-induction of germ cell aplasia was determined. Pregnant mice were dosed orally on days 8-12 of gestation with a benzidine-, dimethylbenzidine-, or a dimethoxybenzidine-based dye. The testes of male offspring exposed to the benzidine-based dyes Congo red, diamine blue or chlorazol black E were small and contained tubules completely devoid of germ cells. The dimethylbenzidine-based dyes, trypan blue, Evans blue and benzopurpurin 4 B, and the dimethoxybenzidine-based dye, Chicago sky blue, were without testicular effects. Azole diazo component 48, a dimethoxybenzidine congener, and two other diazo dyes, naphthol blue black and Sudan III also had no effect on the testes of the male offspring. The results of these studies demonstrate that only the benzidine-, but not dimethyl- or dimethoxybenzidine-based dyes produce persistent hypospermato-genesis in the testis of the mouse as a consequence of fetal exposure.