The in vitro production of β 2 -M by B-CLL cells from 27 patients was investigated. In all cases, low spontaneous β 2 -M release was observed. The production of β 2 -M was enhanced to various extents when induced with 13 different stimulants and their combinations including IL-2, TNFα, SAC and TPA. β 2 -M release was 3.8-fold (range from 1.9 to 9.2-fold) in cultures stimulated with TPA (10 ng/ml), compared with the spontaneous release, and even faster if TNFα or IL-2 were added. A strong correlation was revealed between β 2 -M production and the total protein synthesis of leukaemic cells when the latter was assessed using 1 4 C-l-leucine incorporation. Hence, both β 2 -M release and leucine incorporation are promising activation markers for CLL B-lymphocytes.