Lymphoid cell lines could easily be directly established in vitro from spontaneous thymomas of aged AKR mice. The established lines all carried the Thy-1.1 antigenic marker of T lymphocytes. They differed markedly, however, from one another in several respects. Thus, they showed high or low quantity of Thy-1.1 antigens per cell, high or low sensitivity to cortisone and thymidine, as well as differing morphology and growth rate. The observed heterogeneity suggests that such AKR thymoma cell lines may provide a valuable tool to study T lymphocyte properties and the biology of the AKR thymoma, in particular non-immune and immune mechanisms influencing thymoma cell growth.