Abstract The effects of rabbit T lymphocytes on rabbit aortic smooth muscle cell (SMC) phenotype and proliferation were investigated in vitro. SMCs seeded at confluent density in primary culture had a volume fraction of myofilaments (V _v myo) of 49.8±2.6% after 3 days of culture, not significantly different from that of freshly dispersed cells (V _v myo, 54.1±2.1%). Sister cultures of SMCs to which Concanavalin A–activated T lymphocytes or T lymphocyte–conditioned medium was added had significantly lower V _v myo (35.5±2.2% and 31.6±2.3%, respectively) at the same time point. We have previously shown that a decrease in V _v myo could be induced by the heparan sulfate–degrading activity of living macrophages and by commercial preparations of heparinase. While activated T lymphocytes also completely degraded heparan sulfate–rich ³⁵ S-labeled extracellular matrix (an effect inhibited by the addition of 10 μg/mL heparin), no heparanase-like activity was detected in T lymphocyte–conditioned medium, indicating that for this cell type SMC phenotypic change is induced by a different mechanism. Incubation of the T lymphocyte–derived cytokine interferon gamma (IFN-γ) with freshly isolated rat SMCs caused a significant reduction in V _v myo at day 2 in primary culture from 54.3±2.1% (control) to 35.4±3.0%. Furthermore, a neutralizing antibody specific for IFN-γ removed the effect of T lymphocytes and medium conditioned by them, thus positively identifying IFN-γ as the T lymphocyte factor responsible for this activity. T lymphocyte–conditioned medium was mitogenic for passaged (low V _v myo) SMCs. Although SMC proliferation was inhibited by exogenous IFN-γ, two other T lymphocyte products, granulocyte-macrophage–colony stimulating factor and tumor necrosis factor–β, were found to stimulate proliferation, while interleukin-2 and interleukin-6 had no effect. It was concluded that T lymphocytes, by inducing SMC phenotypic change and stimulating proliferation, may play an important role in atherogenesis.