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.