A factor capable of effecting passive transfer in vivo of delayed hypersensitivity to tuberculin to recipients that are tuberculin negative was isolated from the dialyzate of disrupted leukocytes of tuberculin-positive individuals. After this factor was incubated with cultures of peripheral leukocytes from tuberculin-negative individuals, the addition of purified protein derivative of tubercle bacilli resulted in leukocyte stimulation similar to that observed after addition of purified protein derivative to leukocytes from tuberculin-positive individuals.