Rabbit kidney cell monolayers infected with herpes simplex virus (HSV) were incubated with leukocytes from rabbits immunized with complete Freund's adjuvant. When the leukocytes were exposed to tuberculin purified protein derivative (PPD), viral replication and plaque formation were markedly inhibited. Similarly, when leukocytes from animals immunized with HSV were exposed to UV-inactivated HSV, viral replication was markedly inhibited. Exposure of leukocytes from unimmunized animals or animals immunized with incomplete Freund's adjuvant to UV-inactivated virus or PPD produced relatively little inhibition of viral replication. Examination of supernatant fluids from stimulated cultures revealed a soluble mediator that had the properties of interferon. Interferon production was detected within several hours after exposure of sensitized leukocytes to antigen. Supernatant fluids from as few as one sensitized leukocyte per 200 rabbit kidney cells inhibited HSV replication by over 90%. These findings support the concept that the cellular immune response to HSV consists of two phases: an immunologically specific antigen recognition phase, and a nonspecific effector phase that stops HSV spread by generating interferon.