Immunofluorscence studies have shown that injection of lymphocytes from either the intestinal lamina propria (LP) or from the Peyer's patches (PP) into irradiated (1000-1250 rad) allogeneic rabbits leads to the differentiation and proliferation of IgA containing cells in the spleen, mesenteric lymph node (MLN) and intestine by day 6 and to a lesser extent by day 4. In contrast, few IgA containing cells were seen in irradiated animals not given lymphocytes or given popliteal lymph node cells. Transfer of PP lymphocytes resulted in statistically greater numbers of IgA containing cells in the recipient MLN or spleen than did transfer of LP lymphocytes. In the PP of recipient rabbits given either LP or PP lymphocytes, intercellular IgA was abundant and more IgA containing cells were seen near the PP than in sites distant from PP. These results show that IgA precursor cells are present in the intestinal LP as well as in the PP. The nature and distribution of the IgA precursor and the characteristics of the IgA repopulation are discussed.