Interactions between human lymphocytes and target cells are highly variable. Lymphocytes may enter fibroblasts, establish cytoplasmic bridges, and draw material through these bridges from the fibroblasts; these lymphocytes may undergo divisions within the fibroblasts, while the fibroblasts vacuolize and die. In this interaction, the fibroblasts may serve as feeder cells to the lymphocytes without any immunological principle involved. Small (presumably immune T) and large (presumably NK) lymphocytes of tumor-bearing patients lyse either autologous, or allogenic, or both autologous and allogeneic tumor cells. Serum factors can block or intensify these reactions. Healthy donors frequently yield lymphocytes cytotoxic to tumor cells. Serum factors blocking the cytotoxicity of patients' lymphocytes frequently fail to block the cytotoxicity of healthy donors' lymphocytes.