Lines of rat T lymphocytes responsive to the basic protein of myelin (BP) or to the purified protein derivative of Mycobacterium tuberculosis (PPD) were inoculated i.v. into recipient rats. As reported previously, the anti‐BP line cells, but not the anti‐PPD line cells spontaneously accumulated in the central nervous system and caused encephalomyelitis. However, the anti‐PPD line cells could be induced to enter the brain and cause bystander encephalitis by intracerebral inoculation of PPD. Anti‐PPD or anti‐BP line cells could mediate delayed‐type hypersensitivity skin reactions or bystander arthritis elicited by specific antigen. The lines did not cause specific cytolysis in vitro. Susceptibility to delayed‐type hypersensitivity or bystander disease was long lasting in rats inoculated with anti‐PPD line cells, while rats inoculated with anti‐BP line cells were susceptible for only a few days. Thus, lines of T lymphocytes can mediate a variety of pathological reactions directed by the presence of specific antigen, self or foreign.