Repopulation of the IgA plasma cells in the gut lamina propria was studied by lymphocyte transfers between histocompatible, immunoglobulin allotype congenic mice (CB-20 and BALB/cJ). Among the lymphocytes transferred, Peyer's patch lymphocytes showed the highest efficiency in the repopulation. The donor IgA plasma cells appeared suddenly in the gut lamina propria in substantial numbers at day 8, increased exponentially, and repopulated essentially tne entire gut lamina propria at days 12 to 14. By sequential transfer of lymphoid cells, lymphoid cell culture, splenectomy, and jejunum-shielded total body irradiation of the recipient mice, the majority of the patch IgA precursors were found migrating directly to the spleen rather than to the gut lamina propria. The patch IgA precursors resided in the spleen for at least 5 days and then migrated to the gut lamina propria, where they further divided and matured into IgA plasma cells. However, the spleen is not a completely obligatory tissue in the migration route because the repopulation was only partially reduced when patch cells were transferred into splenectomized recipients. In the splenectomized recipients, the patch IgA precursors did not migrate directly to the gut lamina propria either, which suggests that indirect homing to the gut lamina propria is a characteristic of the patch IgA precursor cells.