A number of different biological properties have been ascribed to the hormone-like protein interleukin 2 (IL-2). However, the most salient feature of this lymphokine is its ability to sustain the long-term proliferation of T-cells from humans and mice. Reported herein are the results of studies demonstrating the isolation of growth factor-independent cell lines from the long-term IL-2-dependent murine T-cell line CTLL-2 that is used frequently as the source of target cells in IL-2 bioassays. Sustained log-phase growth of these T-cells in vitro has been achieved using Petri dishes of polymethylpentene; growth could not be sustained in similar dishes of glass, untreated polystyrene, polystyrene that had been treated for cell culture, or polycarbonate. The IL-2-independent line grew as a T-cell lymphoma when injected i.p. into pristane-treated, but not untreated, syngeneic C57BL/6 mice. In contrast, cells from the IL-2 parental line CTLL-2 did not grow in vivo. Characterization of the IL-2-independent lines propagated in vitro (denoted as line CEC) or in vivo (denoted as line CEP) demonstrated that they retained their dependency for 2-mercaptoethanol and expressed phenotypic profiles of their parental line CTLL-2 (Thy 1.2+, Lyt-1-; Lyt-2-). Isolation of an IL-2-independent T-cell lymphoma from a CTLL-2 line obtained from another investigator using a protocol that has proven reproducible under carefully controlled laboratory conditions and defined phenotypic traits of the syngeneic T-cell isolates provided evidence that the tumors were not a cross-culture contaminant arising as a result of a laboratory accident. Moreover, karyotypic analysis using a quinacrine:Hoechst banding technique revealed similar marker chromosomes in the IL-2-dependent and -independent lines. IL-2-independent lines have also been established from the IL-2-dependent murine T-cell line CT-6. Accordingly, the results of these studies suggest that, during prolonged cultivation that has included exposure to crude IL-2 preparations known to contain phorbol ester, possibly viruses, and other contaminants, the IL-2-dependent lines have developed subpopulations that are thought to have undergone malignant transformation of unknown etiology to generate IL-2-independent murine T-cell lymphomas that can be passaged repetitively either in vitro or in vivo.