The stimulation of cells in gut associated lymphoid tissue (GALT) by intestinal antigens can result either in immunity or tolerance to that antigen. The factors that determine which effect predominates are not understood, but the answer seems to lie in a better understanding of the cellular interactions and regulatory mechanisms in GALT. Induction of an immune response in GALT involves the same macrophage-T cell, T cell-T cell, and T cell-B cell interactions that have been described in other lymphoid tissues. Regulatory T cells have now heen shown to play an important role in controlling the immune response to intestinal antigens. The presence of helper T cells specific for the IgA isotype in Peyer's patches may partly explain the old observation that the intestinal route is preferential for this antibody class. The stimulation of suppressor T cells in GALT is responsible for many instances in which tolerance rather than immunisation has followed antigen feeding. Although there is experimental evidence supporting the idea that mucosal IgA immunity and systemic IgG tolerance can occur concomitantly after antigen feeding. recent data obtained after the feeding of a variety of protein antigens indicate that this is not the usual result. More commonly either immunity or tolerance occurs concomitantly in both mucosal and systemic systems after antigen feeding, suggesting that the suppressor cells mediating oral tolerance also suppress mucosal IgA responses. © 1985 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.