The proliferative and differentiative potential of Peyer's patch, peripheral blood, and popliteal lymph node cells was assessed by allogeneic cell transfer followed by quantitation of donor immunocytes by immunofluorescence. It was found that Peyer's patches are a highly enriched source of cells which have the potential to proliferate and differentiate into IgA-producing immunocytes and that the Peyer's patch cells are far more efficient in seeding the gut of irradiated recipient rabbits with donor cells that give rise to immunoglobulin-producing cells than cells from peripheral blood or popliteal lymph nodes.