Incubation of the tolerant cell population with anti- serum and complement reversed the suppressor effect. Furthermore, the addition of purified T cells from normal donors restored the capacity of the anti- serum-treated tolerant cells to transfer an adoptive response to FG. The existence of FG-reactive B cells was supported by the demonstration of normal numbers of antigen-binding cells in the spleen and thoracic duct lymph from tolerant animals. Moreover, the formation of caps by these cells implied that they could bind antigen normally.

These experiments provided direct evidence for the existence of suppressor T cells in the tolerant population. Further evidence was derived from examination of the effect of antigen "suicide". Tolerant spleen cells were treated with radioactive FG under conditions known to abrogate T-cell helper function. When these cells were transferred together with normal spleen cells into irradiated hosts, suppression of the primary adoptive response to FG was no longer observed. Inhibition of an adoptive secondary response to FG was obtained by transferring tolerant spleen cells with primed B cells provided high doses of tolerant cells were used. By contrast low doses exerted a helper rather than a suppressor effect in this system.