The electrophoretic mobility test (EMT) is an in vitro assay for demonstrating cellular immunity. In the presence of tumor antigens lymphocytes of tumor patients liberate lymphokines, which reduce the charge of indicator particles resulting in a measurable reduction of their electrophoretic mobility. Lymphocytes of 174 patients were tested by EMT. The antigens used were a basic myelin protein termed encephalitogenic factor (EF) and a 3M KCl extract from melanoma tissue. In 91% of the cancer patients there was a positive lymphocyte response. In contrast to this the controls and non-malignant diseases showed a positive result in only 8.7% of the cases. Using the 3M KCl extract from melanoma tissue as tissue as antigen 1 of the benign controls, 3 patients with non-malignant diseases and none of the 49 patients with malignant diseases reacted positively, whereas in the melanoma group 86% showed a positive lymphocyte response. The results show the possibility of demonstrating tumor specific immune reaction in the EMT.