The in vitro absorption by rat jejunal and ileal gut sacs of soluble antigen-antibody complexes and of antigen alone was compared. Complexes prepared in 2-fold antibody excess were absorbed in significantly smaller quantities than was antigen alone. Complexes prepared in 50-fold antigen excess were absorbed by jejunal gut sacs in quantities equivalent to that of antigen alone, whereas the absorption of such complexes by ileal sacs was somewhat decreased compared to that of antigen. There was less radiolabeled antigen tightly bound to the intestinal mucosa of gut sacs exposed to complexes prepared in antigen or antibody excess compared to antigen alone. Complexes prepared in antibody excess appeared to stimulate secretion of mucus and complexes were associated with the mucus fraction. It was suggested that the large size of complexes prepared in antibody excess may result in their trapping in the mucus coat of the gut, thereby preventing contact with the surface of the enterocyte from whence uptake by pinocytosis is initiated. In addition, complexes appear to be shed from the surface with mucus by a mechanism still to be elucidated.