The surface phenotype of Peyer's patch germinal center lymphoid cells in the mouse is described. It is confirmed that most germinal center lymphocytes bind high levels of peanut agglutination (PNA), a lectin with specificity for terminal galactosyl residues. It is shown that germinal center lymphocytes can be identified in cell suspensions as a discrete PNAhi population distinct from other B cells, plasma cells, and most T cells, which bind only low levels of PNA. Using fluorescence-labeled PNA as a marker in dual fluorescence studies, we found that the majority of Peyer's patch germinal center cells are B lymphocytes: PNAhi Peyer's patch cells express B220, the B lineage-specific form of the T200 family of molecules, as well as low levels of surface Ig. They do not express the T cell-lineage antigens Thy-1, Lyt-1, or Lyt-2 (only 1 to 3% positive). They bear lower levels of H2-K than PNAlo B cells, but two to three times the level of surface I-A-encoded determinants. A discrete but variable subpopulation of PNAhi Peyer's patch cells bear ThB in AKR/c mice, but BALB/c PNAhi lymphocytes are ThB-. About 10 to 30% bear surface IgM or IgG, but in contrast to essentially all PNAlo B lymphocytes in this site, they express no detectable surface IgD. The majority of Peyer's patch germinal center cells bear surface IgA, and this IgA is allelically excluded in F1 mice, indicating it is synthesized by the germinal center cells themselves. In fact, germinal centers contain most of the IgA-bearing cells in Peyer's patches (70 to 85%). These findings lend considerable support to the concept that germinal centers in Peyer's patches are the site of generation of precursors of the IgA-secreting plasma cells that characterize mucosal immune responses, and also suggest that germinal centers may play an important role in the process of heavy chain class switching.