The fate of mesenteric lymph node lymphoblasts labeled with either [125I]iododeoxyuridine or [3H]thymidine can be studied after intravenous transfer into syngeneic mice both by measurement of radioactivity in various organs and by combined immunofluorescence and autoradiography of recipient tissues. Many of the lymphoblasts home to the lamina propria of the small intestine within hours of transfer; of these, many visibly secrete IgA. To determine whether the cells that will ultimately secrete IgA are already committed to IgA synthesis before their arrival in the gut, mesenteric lymph node cell populations were treated with various class-specific antisera to mouse immunoglobulins before transfer. Treatment with antiserum to IgA, plus complement, reduced the fraction of injected label recovered from the recipients' intestines, and also reduced the proportion of donor (labeled) cells containing IgA. We conclude that mesenteric lymph nodes are probably the principal source of IgA-secreting plasma cells in the lamina propria of the gut, and that the cells become committed to IgA synthesis and develop cell surface IgA before emigrating. This IgA is apparently synthesized by the cells that bear it since it is not removed by extensive rinsing at 37 degrees C, a maneuver that elutes passively adsorbed immunoglobulin.