Recently we described a new monoclonal antibody, termed LAK1, which recognizes a 120-kDa surface antigen that is expressed on virtually all LGL and LAK precursors and effectors. In the present study we describe a second mAb, termed LAK2, which was derived against cloned LAK cells. The LAK2 mAb, similar to the LAK1 mAb reacts with a subset of peripheral blood lymphocytes which includes the precursors of LAK cells. In addition, among IL2-activated peripheral lymphocytes, this antibody defines cells displaying LAK activity. The expression of the LAK2 molecule on PBMC was analyzed by two-color cytofluorometric analysis in comparison with the expression of both T cell and LGL markers. We show that most resting LAK2+ cells lack surface expression of CD3, whereas nearly 60% express CD2 antigen. Moreover, all CD16+ and CD56 (NKH1)+ lymphocytes coexpressed both LAK2 and LAK1 antigens. Morphological analysis of LAK2+ lymphocytes indicated that the majority of these cells was represented by LGL. Thus the expression of the LAK2 molecule on LGL-enriched populations was compared by two-color cytofluorometric analysis to that of other known LGL markers such as CD16, CD57 (HNK1), and LAK1. Most LGL coexpressed LAK1, LAK2, CD16 and CD57 antigens Finally, the surface molecule recognized by LAK2 mAb is composed of two chains with apparent molecular masses of approximately 110 and 140 kDa.