Treatment of cancer patients with interleukin-2 has resulted in clinical regression of metastatic disease. The ability to predict which patients will respond to therapy could result in more efficacious and possibly less toxic treatments. Hence, attempts have been made to correlate patient parameters or in vitro characteristics with clinical response to immunotherapy. Some patient factors have been identified as prognostically favorable. However, parameters modulated by interleukin-2 have not been predictive of clinical outcome. Exciting new evidence does exist regarding in vitro characteristics of tumor-infiltrating lymphocytes. Measurement of the cytolytic ability of melanoma tumor-infiltrating lymphocytes may enhance our understanding of the immune-mediated antitumor response and lead to a new generation of selective immunotherapies.