Summary Peritoneal exudate cells from normal guinea pigs were passively sensitized by various antigen-antibody complexes and studied in the capillary migration system. The normal migration of cells from the capillary tubes was inhibited by red cell-IgG complexes but not by red cell-IgM complexes. The degree of inhibition depended on the dilution of antibody used to sensitize the red cells and the ratio of sensitized red cells to peritoneal exudate cells. The addition of keyhole limpet hemocyanin (KLH) to chambers containing normal peritoneal cells in the presence of anti-KLH antiserum also produced inhibition of migration. In these experiments, the inhibition was proportional to the amount of antigen added to the cultures.