The studies presented here have focused on the important question of reversibility of inactivation of DNP-specific B lymphocytes induced by the DNP derivative of the copolymer of D-glutamic acid and D-lysine (D-GL). In so doing, we have analyzed the capacity of a strong T-cell stimulus, such as that provided by the allogeneic effect, and of gentle enzymatic treatment with trypsin to alter, prevent, or reverse the tolerance induced by DNP-D-GL. Under experimental conditions in which DNP-specific B lymphocytes were exposed first to the tolerogenic molecule, and rendered markedly unresponsive by such exposure either in vitro or in vivo, subsequent exposure to an allogeneic effect failed to appreciably reverse or alter the tolerant state. This contrasts directly with the capacity of DNP-D-GL to serve as a stimulus for DNP-specific B lymphocytes when the critical moment of specific binding occurs subsequent to the development of an allogeneic effect. In another series of experiments, the effects of enzymatic treatment with trypsin on the tolerant B-cell population were found to vary depending on the stage of tolerance at which such treatment was performed. Thus, when exposure of cells to DNP-D-GL for a relatively short time in vitro is carried out at low temperature (4 degrees C), the development of tolerance can be interceded by immediate trypsinization. In contrast, cells exposed to DNP-D-GL for longer periods of time and/or at 37 degrees C were not reversed to responsiveness by trypsinization. These data were interpreted to indicate that: (a) the effect(s) of trypsin in reversing (or preventing) tolerance at the cellular level does not depend necessarily on the susceptibility of the tolerogenic moiety to the action of the enzyme, and (b) the generation of the tolerance-inducing signal involves metabolic cellular processes that can be delayed somewhat by low temperature leaving such cells relatively more susceptible to intercedent manipulations such as trypsinization. Taken collectively, therefore, the evidence obtained in these studies reinforces the concept of central tolerance in B cells induced by DNP-D-GL as reflecting sub- or intracellular inactivating events.