The expression of the Thy-1 ''differentiation alloantigen'' was analyzed in rat tissues of two strains. Cerebrum was shown to express an amount of Thy-1 equivalent to the thymus. The cerebellum, brain stem and spinal cord expressed approximately one-half this amount, and the olfactory bulb expressed 10- to 15-fold less. Of the peripheral nervous system tissues examined, only cauda equina expressed substantial amounts of Thy-1, although minor amounts were found on the lumbar roots and trigeminal nerve. Kidney, spleen, liver, salivary gland and testes expressed little or no Thy-1. Several cell lines derived from nervous system tumors were also examined. RN-1, a line established from a rat peripheral neurinoma, expressed Thy-1 at levels comparable to thymus.