Using immunocytochemical phenotyping, the authors assessed the origin of malignant cells in a patient with malignant centroblastic lymphoma. The lymphoma cells expressed the thymocytic surface molecule CD1, pan T-lymphocytic markers CD2, CD5, CD6, CD7 with concurrent absence of complex CD3 and receptors for antigen on T lymphocytes. The proliferating activity of malignant cells could not be controlled by intensive cytostatic therapy.