T-lymphocyte subgroups and the percentage and the activity of Human Natural Killer cells (HNK) were investigated by the authors in their 24 patients suffering with low grade and 24 with high grade malignancies of non-Hodgkin lymphoma (NHL). According to their observations, the ratio of CD3, CD4, Leu-7, and HNK cells as well as the release by HNK cells of 51Cr bound to target cells decreased, depending on the pathological stage. The tests were performed with monoclonal antibodies. A significant change could also be observed in the reactivity of bone marrow cells to monoclonal sera, which showed complete correspondence in character with the behavior of lymphocytes isolated from peripheral blood. No significant changes could be observed in the quantitative relations of immunoglobulins. Discussing the supposed immunological relation in detail, the role of plasma factor that reduces the formation of T-lymphocytes, is assumed to have primary importance in this phenomenon.