A Mab set was used to study immunological phenotype of lymphocytes of five patients bearing stage IV malignant melanoma and its changes developing in the course of interleukin-2 (Euro cetus) immunotherapy. Patients were shown to have lowered counts of T cells (mainly due to T-helper/inducers), B cells and lymphocytes expressing adhesion and activation molecules as well as abnormal immunoregulatory cell ratio. Two patients receiving interleukin-2 achieved a response. Follow-up of antigen expression revealed a rise in natural killer and immunoregulatory T lymphocyte levels in the responders whereas activation antigens and B cell levels were increased in all the patients.