Some salivary epitheliomas induced in C3H/Bittner (C3H/Bi) mice by neonatal injection of the fifth in vitro passage of parotid tumor agent isolate of mouse polyoma virus are infiltrated with T lymphocytes having an immature phenotype. Serially transplantable tumor lines have been obtained from primary salivary epitheliomas, and some of these secondary tumors are also infiltrated with variable proportions of immature T cells. A subline has been selected which, in syngeneic hosts, becomes richly infiltrated by immature lymphocytes, as defined by bearing both CD4 and CD8 surface markers. While some sublines produced this infiltrate, others did not. There is no relationship between the extent or nature of the T-cell infiltrate and the passage number (generation) of the tumors or the residence period of the transplanted tumors in the host. The results strongly suggest that subsets of salivary epitheliomas resemble the thymic microenvironment sufficiently that T-lymphocyte maturation, at least in part, proceeds readily.