Our studies have been directed to determine the environmental influence of bacterial endotoxin from the indigenous Gram-negative flora on gut-associated lympho-reticular tissue (GALT), e.g., Peyer's patches (PP) and the IgA response. In the present study, we have compared LPS responsive and nonresponsive mice for induction of systemic unresponsiveness to antigen given for prolonged periods by the oral route (oral tolerance). Daily gastric intubation (GI) of LPS-responsive C3H/HeN mice with sheep erythrocytes (SRBC) for 2 wk resulted in oral tolerance, whereas similar treatment of LPS-nonresponsive C3H/HeJ mice primed these animals for anamnestic responses. In vitro studies demonstrated that spleen cells from C3H/HeN mice given SRBC either by GI or by GI followed by a single i.p. administration of SRBC were unresponsive to this antigen. On the other hand, spleen cells from similarly treated C3H/HeJ mice yielded significant direct (IgM) and indirect (IgG1, IgG2, and IgA) responses to SRBC. Prolonged GI of C3H/HeJ mice with SRBC did not result in oral tolerance, although lower responses were seen after GI of SRBC for longer periods (28 to 60 days). Hybrid (F1) mice from crosses between C3H/HeN and C3H/HeJ animals gave low but significant splenic anti-SRBC PFC responses to systemically administered antigen after daily GI or SRBC for a 2-wk period. However, when SRBC was given by GI for longer periods, F1 mice gave somewhat lower responses, a pattern similar to that seen with C3H/HeJ mice. When C3H/HeN mice were first given SRBC by i.p. administration followed by daily GI for 2 wk, oral tolerance was induced, suggesting that systemic unresponsiveness occurs even in the presence of memory cells. The cellular basis for induction of oral tolerance was suggested by the finding that splenic T cells from C3H/HeN mice exhibited significant T suppressor cell activity. On the other hand, T cells from spleen of identically treated C3H/HeJ mice exhibited mainly T helper cell activity. These results suggest that oral administration of a thymic-dependent antigen to LPS-responsive mice induced a T suppressor cell population that suppressed responses to systemically administered antigen. LPS-nonresponsive mice lack this T suppressor cell pathway and continually respond to oral and systemic administration of antigen.