A novel activation-dependent lymphocyte cell-surface antigen which is recognized by a MoAb, BL-Ac(F2), is described. Although not found on resting lymphocytes the antigen is induced rapidly within 2-4 h following stimulation of the cells using mitogens or antibodies against the T-cell CD3 antigen and sIgM on B cells, respectively. Immunoprecipitation and Western Blotting indicated that the MoAb recognizes a molecule in a range of 78-85 kDa. Beyond its activation-dependent expression on lymphocytes the antigen was detected also on myelo-monocytic cells. Expression kinetics and cellular distribution of this molecule suggest that it is distinct from previously described activation-dependent cell-surface antigens such as CD69, CD25 and 4F2.