A model of unresponsiveness to human gamma-globulin (HGG) which is maintained in the absence of demonstrable suppressor cells has been described. A/J mice were tolerized with deaggregated HGG purified from a variety of sources. The spleen cells from these tolerized mice were assessed for their ability to suppress the response of normal spleen cells to HGG when transferred into lethally irradiated mice. All of the HGG preparations obtained from commercial sources as Cohn fraction II of pooled, outdated plasma induced suppressor cells to HGG, although not of equal magnitude. However, suppressor cells could not be demonstrated in the spleens of mice tolerized with deaggregated HGG purified from the plasma of a healthy individual. This inability to detect suppression was independent of the method of purification of the HGG and of the time of assessment of the putative suppressor cells after tolerization. Similarly, deaggregated HGG isolated from an IgG1 lambda-myeloma protein induced unresponsiveness to HGG but did not stimulate demonstrable suppressor cells. These data suggest that suppressor T cells are not involved in the maintenance of tolerance to this antigen, although they may play a regulatory role in the immune response to HGG. Support for this concept was obtained by assessing the duration of unresponsiveness in the T and B lymphocytes of mice tolerized with the various HGG preparations. Mice tolerized with the HGG preparations that stimulated little or no suppression were among the last to recover responsiveness. Indeed, there was no consistent correlation between the level of suppressor cell activity and the degree of unresponsiveness in either the splenic T or B lymphocytes. Thus, although certain HGG preparations may provide a tool for the generation of antigen-specific suppressor T cells, the utilization of these suppressive preparations may be inappropriate for the investigation of the mechanisms of the induction and maintenance of the unresponsive state.