When organ culture techniques were applied, biopsies of rabbit jejunal mucosa were cultured for 24 hr with maintenance of mucosal architecture. Moreover, villous epithelial cells and most lamina propria lymphoid cells were structurally normal although crypt epithelial cells often showed focal degeneration after 24 hr. Incorporation of radiolabeled leucine and glucosamine into protein and glycoprotein was linear with time. After 3 to 6 hr of incubation, radiolabeled protein was secreted steadily into the culture medium during the subsequent 12 to 24 hr. De novo protein synthesis and secretion were inhibited by cycloheximide. Biopsies cultured in 14C-labeled leucine secreted intact radiolabeled secretory IgA into the culture medium. Secretory IgA-14C was purified from concentrated medium by sequential column chromatography. This material uniformly gave a single radioactive precipitin arc by immunodiffusion and Immunoelectrophoresis with specific antibodies against rabbit secretory IgA, secretory component, or α chain. Thus, when maintained in organ culture, biopsies of rabbit jejunal mucosa synthesize and secrete protein under steady state conditions for 24 hr and secrete intact newly synthesized secretory IgA into the culture medium. Organ culture offers considerable advantage over all previously described intestinal preparations since in vitro synthesis and secretion of macromolecules by intestinal mucosa can be studied for significantly longer periods.