Several soluble factors have recently been associated with the proliferation and differentiation of thymus-derived lymphocytes. One of these factors present in medium conditioned by T cell mitogen-stimulated lymphocytes has the ability to promote the long-term culture of normal and antigen-specific cytotoxic T cells. We report a method to test for this proliferative stimulus in the form of a sensitive microassay based upon the tritiated thymidine incorporation of continuous murine tumorspecific cytotoxic T cell lines (CTLL). The microassay requires microliter quantities of sample fluid and is amenable to quantitative analysis. This highly reproducible, quantitative assay for T cell growth factor (TCGF) has allowed investigation as to the kinetics of TCGF generation and has revealed that T lymphocytes are required for its production. Further investigation has supported the notion that this nonspecies-specific factor is actively removed from tissue culture medium by the proliferation of either T cell mitogen-activated lymphocytes or CTLL.