A study was entered upon to evaluate the immunological reactivity of adult BALB/c (H-2d) mice force fed either 104, 106, or 108 DBA/1 (H-2q) lymphoid cells shortly (<12 hr) after parturition. When these mice were 7 weeks old, their immunological responsiveness was tested with DBA/1 skin allografts. Mice receiving 104 allogeneic lymphoid cells demonstrated an accelerated graft rejection, whereas those that received 106 or 108 allogeneic lymphoid cells displayed an allograft median survival time (MST) that was not significantly different from that of the littermate controls. Three of 30 and 2 of 14 mice force fed 106 and 108 allogeneic lymphoid cells demonstrated a prolonged survival of skin allograft ranging from 22 to 55 days. None of these mice were observed to have symptoms of runting disease while infants, although runting disease did occur in the force-fed neonates and was greatest in those receiving 108 allogeneic lymphoid cells. However, the frequency of runting disease was not as great as in those neonates either nursed from birth on DBA/1 mothers or injected i.v. with 5 × 106 DBA/1 lymphoid cells. The results of this study suggest that the ingestion of lymphoid cells by suckling can influence the immunological responsiveness of an adult.