Human lymphocytes stimulated in vitro by allogeneic small cell lung carcinoma cell lines did not show any significant cytolytic activity against the stimulator tumor cells. However, a high level of lysis was observed when both stimulator and target small cell lung carcinoma cells were pretreated with interferon gamma, which increased considerably the expression of major histocompatibility class I molecules by these cells. The demonstration that small cell lung carcinoma cells can be lysed by cytolytic T lymphocytes, suggests that it will be feasible to study the autologous T cell response of patients against this tumor.