Female BALB/c (H-2d, Mlsb) mice alloimmunized prior to and during syngeneic pregnancy with DBA/2 (H-2d, Mlsa) splenocytes gave rise to offspring with severely reduced responsiveness in adult life to DBA/2 stimulation in vitro mixed lymphocyte cultures. The offspring of the hyperimmunized mothers were also tolerant to neonatal challenge with large numbers of DBA/2 splenocytes, which resulted in runting disease of control neonatal BALB/c mice. Both challenged and unchallenged offspring of the immunized BALB/c mothers were hyporesponsive to DBA/2 but both stimulated and responded to normal BALB/c lymphocytes, indicating alteration in their T-cell repertoire. There was no reduction in the V beta 6-positive thymocyte subpopulation in the challenged or unchallenged offspring of the alloimmunized BALB/c mothers compared to normal controls, suggesting that hyporesponsiveness to DBA/2 is not due to thymic deletion of Mlsa-responsive clones. Examination of the T-cell subset composition of the hydrocortisone-resistant thymocytes and peripheral lymphocytes of the challenged and unchallenged groups of experimental mice revealed large increases in the percentage of Lyt-2+ T cells, sometimes accompanied by a decrease in the L3T4+ T-cell subset compared to age-matched control BALB/c. Lymphocytes from the hyporesponsive mice specifically suppressed the proliferative responses of control BALB/c to DBA/2 but not to AKR. The data indicate that maternal hyperimmunization can induce tolerance by a mechanism involving intrathymic selection of suppressor cells which can be combined with a negative selection of helper cells.