The transient presence of suppressor T cell (Ts) activity in high-dose tolerance to human gamma globulin (HGG), and its (apparent) absence in low-dose tolerance, have been advanced as strong evidence against the concept that Ts play an important role in maintenance of immunological unresponsiveness. To analyze this question, CBA mice were exposed to high or low doses of deaggregated HGG (dHGG) and later challenged with HGG in immunogenic form (aHGG); their capacity to mount a primary or secondary suppressive response was assessed in an adoptive hapten-carrier system. Primary suppression reached a maximum 7 d after high-dose tolerance induction and gradually waned thereafter, being no longer detectable by day 30-35. Subsequent challenge of tolerant mice with aHGG, however, led to a rapid reactivation of suppression that bore the hallmarks of an anamnestic secondary response, and this effect was still demonstrable 135 d after tolerance induction. It was also shown that a single low dose of dHGG was capable of generating memory for suppression despite the absence of detectable primary suppression, indicating that the latter is not a prerequisite for induction of memory cells. The results were interpreted as indicating that tolerance, like immunity, is a manifestation of specific immunological memory. If tolerance to self-antigens is maintained by a similar mechanism, it would be expected that memory Ts could be induced during the early stages of fetal development. Mice were therefore exposed to tolerogen in utero by injection of their mothers with dHGG at day 7 of gestation, and were assessed at various times after birth for the capacity to exhibit primary or secondary suppression in adoptive transfer. Nonspecific suppression masked any specific effects during the first 5 wk of life. Antigen-specific, primary suppression was demonstrable subsequently until 10-12 wk of age, and if the animals were challenged with aHGG before transfer an anamnestic secondary suppressive response could be elicited up to 6 mo of age. These observations are consistent with the notion that memory Ts may play an important role in the maintenance of self-tolerance.