The first line of defense against pathogens that enter the host by the oral route appears to involve the gut-associated lymphoreticular tissue-e.g., Peyer's patches (PP). Although animals can readily be immunized by orally administered antigen that mobilizes the secretory immune system, there is a total lack of local antibody synthesis in the PP and the cellular basis for this deficiency remains a mystery. A lymphoreticular cell population, obtained when murine PP were treated with a neutral protease (Dispase), consisted of accessory cells [macrophages (MPhi)] and T and B lymphocytes. In vitro cultures of these PP cell preparations with the thymic-dependent antigen sheep erythrocytes (SRBC) resulted in good anti-SRBC plaque-forming cell (PFC) responses. The time courses of these responses were identical to those seen with spleen cell cultures. Submitogenic concentrations of concanavalin A (Con A) and optimal doses of N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) and lipopolysaccharide (LPS) enhanced in vitro responses of PP cell cultures to SRBC. PP possess fully functional antigen-presenting MPhi because incubation of optimal proportions of splenic T and B cells with purified populations of PP MPhi supported good in vitro immune responses. Murine PP possess all of the necessary elements for an IgA immune response because PP cell cultures derived from mice orally primed with SRBC and immunized with SRBC in vitro gave high IgA anti-SRBC PFC responses. All of the adjuvants tested (LPS, MDP, and Con A) enhanced IgA responses in PP cell cultures from orally primed mice; however, Con A induced the greatest enhancement. These results demonstrate that murine PP possess MPhi capable of accessory cell functions for in vitro immune responses and that oral priming with antigen induces the precursor T- and B-cell populations necessary for IgA responses, that are potentiated by adjuvants, in PP cell cultures. Thus, murine PP possess the lymphoreticular cells required for antibody responses; however, the tissue architecture likely prevents local responses in vivo. The finding that enzymatically dissociated PP contain all of the necessary cellular components for antibody synthesis, whereas the in vivo tissue architecture prevents the complex interactions necessary for this response, suggests that the initial inductive events take place in situ, and additional cell interactions are required for final differentiation of IgA-synthesizing plasma cells to occur at distant mucosal sites.