Comprehensive analysis of skin infiltrate cells, carried out in malignant lymphomas of the skin, has included examinations with the use of monoclonal antibodies (MCA). Basing on the clinical, histologic, and immunologic findings, mycosis fungoides has been diagnosed in 9 patients and T-cellular lymphoblastic lymphosarcoma consisting of T-suppressors in 1. The findings evidence that (1) mycosis fungoides is a tumor consisting of OKT4 cells in the majority of cases; (2) mature T-cell markers are sometimes lost in the course of mycosis fungoides, this being parallelled by the emergence of antigens characteristic of earlier differentiation stages, e. g. corticothymocytic ones; (3) immunologic heterogeneity is a characteristic feature of malignant lymphomas of the skin; this may be explained by both: the tumor progress of the clone and the therapeutic pathomorphosis; (4) use of MCA to a wide spectrum of leukocytic differentiation antigens permits an accurate estimation of the nature of the tumor cells and of the cellular relationships; this helps understand the pathogenesis of malignant lymphomas of the skin and develop new treatment modalities.