The B subunit of heat-labile toxin (LT-B) from enterotoxigenic Escherichia coli has been shown to be a powerful mucosal immunogen. Oral immunization of mice with LT-B revealed that BALB/c (H-2d) and C57BL/6 (H-2b) mice gave high serum IgG and mucosal IgA responses specific for LT-B. However, ALY (H-2b) mice lacking intestinal Peyer's patches (PP) did not respond to oral LT-B with either serum or mucosal antibodies. These results indicate that PP lymphocytes supported both systemic and mucosal immune responses when the antigen was administered orally. Reverse transcription polymerase chain reaction analyses revealed that PP CD4 T cells expressed early Th1-type (interferon-gamma and interleukin [IL]-2) and late Th2-type (IL-4, -5, and -6) cytokine mRNA. These results suggest that the systemic and mucosal antibody responses induced by LT-B are regulated by a cooperation of PP Th1 and Th2 cells.