Our laboratory has previously identified a soluble factor derived from head and neck squamous cell carcinoma that impairs lymphocyte proliferative responses in vitro. This study further investigates the nature of the interaction between these factors and T lymphocytes. The proliferative activity of phytohemagglutinin-stimulated peripheral blood lymphocytes and the Jurkat T-cell line was significantly suppressed (>50%) by the supernatants of 13 (41.9%) of 31 recently explanted head and neck squamous cell carcinoma samples. A characteristic morphologic appearance of these suppressed cells and ladderlike pattern of DNA fragmentation on gel electrophoresis indicated that the suppressive supernatants were inducing or predisposing T cells to apoptotic death. This apoptosis-inducing activity may be similar to that previously described in a suppressive supernatant obtained from an esophageal carcinoma cell line. These results shed further light on the mechanism behind a soluble immunosuppressive factor or factors produced by head and neck squamous cell carcinoma.