Summary: Passive transfer of tuberculin hypersensitivity was accomplished with nonviable cellular constituents of alveolar macrophages of sensitized and challenged rabbits. Although cellular extracts per se had no activity to confer sensitivity on negative recipients, certain fractions obtained from them by electrophoresis, dialysis or treatment with ammonium sulfate, showed apparent activity to confer sensitivity. Thus, “transfer factor” is apparently present in rabbits as well as in man. “Transfer factor” is contained, but in inactive form, mainly in the supernatant fraction separated from particulate components of cells by 20,000 × G centrifugation. After dialysis of this supernatant, “transfer factor” appears in full activity. Dialyzed sera of sensitized and challenged rabbits also showed apparent activity to confer sensitivity, although serum per se had no activity. These facts led us to postulate the existence of a low molecular weight inhibitor. Reasons for these successes in effecting passive transfer with nonviable materials, despite many unsuccessful previous investigations in guinea pigs, have been discussed; in particular, the procedure of “challenge” by injecting BCG 4 days before harvesting the cells is thought to be very important for these positive results.