Guinea pigs were immunized with 2.4 mg of killed tubercle bacilli in adjuvant and Tuberculin Purified Protein Derivative (PPD) was used as test antigen. Peritoneal exudate cell migration inhibition was more marked one year than one month after sensitization. Macrophage cytophilic antibody (detected by rosette formation with antigen coated sheep red cells) was found on the surface of peritoneal macrophages and in sera at one year, but was barely detectable at one month. One year sera as final media in migration chambers inhibited migration of normal peritoneal cells, so did one month sera. Incubation of normal cells in one year sera followed by washing led to migration inhibition. Sera of guinea pigs passively sensitized with the immune sera had haemagglutinating activity against PPD coated sheep red cells. In some of the animals injected with one year sera cytophilic antibody was demonstrable on the surface of peritoneal cells and the migration of these cells was slightly inhibited by antigen. The degree of migration inhibition did not correlate with haemagglutinating and haemolysing antibody titres.