A single oral dose of ovalbumin (Ov) resulted in inhibition of IgE formation in mice subsequently immunized i.p. with Al(OH)3-Ov. Repeated feeding of Ov (on alternate days for 2 weeks) induced the formation of detectable suppressor cells in Peyer's patches and spleen. Suppression was demonstrated by the ability of adoptively transferred Peyer's patch or splenic lymphocytes from Ov-fed tolerant mice to inhibit IgE formation in Ov-immunized syngeneic recipients. Suppressor cells could be induced by feeding mice as little as 100 microgram of Ov on alternate days for 2 weeks. Suppression was specific and Peyer's patch lymphocytes were shown to be more effective suppressors than splenic lymphocytes.