The distribution and differentiation of rabbit Peyer's patch cells were studied in lethally x-irradiated autologou, and allogeneic recipients, 6 days after i.v. transfer. Immunoglobulin Ig-containing cells were detected in both frozen tissue sections and fixed smears of single cell suspensions of spleens using monospecific anti-alpha, -mu, and -iota antisera in direct immunofluorescence. The tissues of lethally x-irradiated rabbits which had not received any cells were almost devoid of Ig-containing cells whereas the intestinal lamina propria of both autologous and allogeneic Peyer's patch cell recipients had approximately equal numbers of predominantly IgA-containing cells. In the spleen of allogeneic recipients, many large colonies of IgA-containing cells were seen and these cells made up 4.6 to 16.9% (mean 8%) of all spleen cell suspension. The spleens of autologous recipients contained fewer and smaller colonies of IgA cells which made up only 0.5% of all cells in spleen cell suspensions. The numbers of IgM- and IgG-containing spleen cells were small in all animals, however, recipients of allogeneic Peyer's patch cells had four to five times as many as either non-reconstituted lethally irradiated rabbits or autologous recipients. These data confirm that rabbit Peyer's patches are relatively rich in precursors of IgA-producing cells and suggest that histocompatibility differences may either potentiate differentiation of IgA production or lead to trapping of allogeneic Peyer's patch cells in the spleen. Although these experiments did not elucidate the mechanisms of differentiation of IgA-producing cells, the fact that no differences were seen between numbers of IgA cells in gut lamina propria after either autologous or allogeneic cell transfer suggest that different mechanisms may be responsible for appearance of these cells in gut and spleen.