The monoclonal antibody (mAb) UN2 was generated upon immunization of a Balb/c mouse with human thymocytes. mAb UN2 recognized an antigen expressed by a subpopulation of human thymocytes and peripheral blood lymphocytes. In thymus, mAb UN2 recognized cortical cells; its expression was higher on CD3bright than on CD3dim thymocytes. This antigen was also detected on peripheral blood granulocytes, monocytes, platelets and on cell lines MOLT4, U937 and KG1. mAb UN2 was submitted to the 5th International Workshop and Conference on Human Leukocyte Differentiation Antigens, Boston, MA, 1993, and was assigned to the CD31. Expression of the UN2-recognized antigen in malignant lymphoid cells from 57 cases of B-cell chronic lymphoproliferative disease and 4 of B-cell acute lymphoblastic leukemia was analysed in flow cytometry. Among the 57 cases of B-cell chronic lymphoproliferative malignancies studied, 49 were classified as B-cell chronic lymphocytic leukemia. These showed high (86 +/- 8%) UN2 antigen expression. In 8 cases of hairy-cell leukemia the percentage of cells reacting with mAb UN2 was 42 +/- 4%; the fluorescence intensity of labelled cells was lower than that displayed by cells of B-cell chronic lymphocytic leukemia and comparable to that of normal lymphoid cells. mAb UN2 could prove useful in analysis of the lymphoid development and diagnostics of B-cell chronic lymphoproliferative disorders.