It is now apparent that the Peyer's patches of some species exhibit structural, functional and developmental heterogeneity. In sheep, for example, the ileal Peyer's patch (IPP) is the primary, antigen-independent site for the generation of the primary immunoglobulin repertoire and consequent production of the systemic B-cell pool. The pig has three distinct Peyer's patches, including an IPP, but the functional status of this organ, as primary or secondary lymphoid tissue, is not clear. Here, we have systematically characterized pig IPP follicular lymphocytes and show that about 90% B cells that are positive for surface immunoglobulin G (sIgM+) and express an immature phenotype characterized by expression of myeloid marker sWC3 (74-22-15) and two molecules recognized by IPP B-cell-specific monoclonal antibodies (F10/4, F12/35). Extensive apoptosis in vivo and in vitro was demonstrated by electron microscopy, immunohistology with TdT-mediated dUTP nick end labelling, DNA analysis and fluorescence-activated cell sorter analysis. Thus, when isolated IPP follicular cells were incubated at 37 degrees in vitro, the majority of them became apoptotic. The few that survived, however, had lost their expression of sWC3, F10/4, F12/35, but showed an increased expression of sIgM and major histocompatibility complex class II indicating that such surviving cells were of a more mature phenotype. Although more T cells were observed in porcine IPP follicles than in sheep IPP, CD3+ cells comprised less than 5% of the IPP follicular lymphocytes. Thus, the results clearly indicate that pig IPP is equivalent to sheep IPP.