Repeated intraperitoneal administration of F9 teratocarcinoma stem cells (first dose of 3.8 X 10(8) + 3 doses of 7.5 X 10(8) cells at two-week intervals) to guinea pigs starting from birth prevented in more than one third of them (in 10 out of 28) the induction of autoimmune aspermatogenesis by subsequent immunization with spermatogenic cells emulsified with FCA. Cytotoxic and immunofluorescent activities against spermatogenic cells were similar in groups of males after tolerance induction and subsequent immunization and of males immunized only. However, these two groups differed substantially in serological activities against F9 cells which were significantly higher in the former group. The results are discussed in connection with the establishment of the blood-testis barrier which may have resulted in the absence of autotolerance towards spermatogenic cell antigens.