GALT contains Tcs cells which protect IgA responses under conditions of oral tolerance. Oral immunization of erythrocyte antigen induces antigen-specific Tcs cells in Peyer's patches which predominantly support IgA responses with minor IgM and IgG responses upon adoptive transfer to tolerized mice. On the other hand, systemically-primed mice possess Tcs cells which convert oral tolerance to only IgM and IgG responses. In general, effector Tcs cells which abrogate antigen-specific oral tolerance are Lyt-1+, Lyt-2-, L3T4-, I-J+ and V. villosa-adherent. Interestingly, these Tcs cells express T3 molecules which suggest the expression of a TCR which may account for their antigen specificity.