Ricin, a toxic lectin from castor beans greatly enhances IgE responses to bee venom phospholipase A2 (PLA2) in high and low IgE responder strains of rat. The increase in IgE is accompanied by a 60% reduction in the number of CD8+ but not CD4+ T cells in the spleen. Optimal enhancement of IgE by ricin occurs when it is given at the same time as the antigen or 24 hr later, suggesting that it acts on cells which were activated as a consequence of immunization. Radio ligand-binding studies with 125I ricin were used to compare the number of ricin binding sites on CD4+ and CD8+ T cells. No difference was seen in either the affinity or the number of receptors for ricin on the CD4+ and CD8+ T cells of unimmunized rats. In contrast, CD8+ T cells taken from rats which had been immunized with 10 micrograms of PLA2 24 hr earlier demonstrated considerably more ricin receptors (3.9 x 10(7) +/- 2.2 x 10(6) binding sites/cell) than CD4+ T cells (3.19 x 10(6) +/- 1.08 x 10(6) binding sites/cell). However the affinity of the receptors for ricin was unchanged. Cytofluorographic analysis with fluorescein isothiocyanate (FITC)-labelled ricin confirmed these observations and indicated that increased ricin binding occurred on a subpopulation of CD8+ T cells. The effect of CD8+ T cells on IgE regulation was investigated by adoptive transfer. 1 x 10(8) highly purified (> 98%) splenic CD8+ T cells collected from Brown Norway rats 3 days after immunization with 10 micrograms of PLA2 were adoptively transferred to naive, syngeneic recipients. The IgE antibody response to PLA2 + A1(OH)3 seen in these animals was reduced by 91%. Adoptive transfer of CD4+ T cells from the same donor animals did not induce suppression and nor did adoptive transfer of CD8+ T cells from animals given both ricin and PLA2. However, when recipients of CD8+ T cells taken from rats immunized with PLA2 were immunized with a different antigen [ovalbumin (OVA)] and A1(OH)3 the IgE antibody response was also suppressed, although to a lesser extent (66%). These results show that co-administration of ricin and PLA2 depletes a subpopulation of ricin-sensitive, early activated CD8+ T cells and that these CD8+ T cells are potent suppressors of the primary IgE response.