A human cell clone (SF-16) displaying strong cytolytic activity against fresh tumor target cells was used for production of murine mAbs against surface antigens expressed by lymphokine-activated killer (LAK) cells and their peripheral blood precursors. The preliminary screening of hybridoma supernatants was performed according to the ability to bind SF-16 cells. Selected mAbs were further analyzed for their reactivity with several T and B cell lines and with peripheral blood T and non-T cell populations. A selected mAb, termed anti-LAK-1, only reacted with some T cell lines and with 15-30% of PBMC. Approximately 10-15% E-rosetting (T) cells and 40-50% E-rosette-negative cells were LAK-1+, as determined by cytofluorometric analysis. As the fluorescence distribution of LAK-1 antigen was clearly bimodal, LAK-1+ and LAK-1- cells could be separated by FACS. Positive cells were composed of large granular lymphocytes (LGL), whereas negative cells were mostly small lymphocytes and monocytes without LGL. After culture in rIL-2, purified LAK-1+ (but not LAK-1-) cells acquired the ability to lyse NK-resistant fresh melanoma target cells. In addition, only the LAK-1+ fraction of PBMC cultured for 5 d in rIL-2 lysed fresh tumor targets, thus indicating that the LAK-1 antigen is expressed also on LAK effector cells. Unlike some other LGL/NK cell markers, LAK-1 antigen is characterized by a stable expression: thus, LAK-1+ cell populations cultured for up to 20 d in rIL-2 maintained the LAK-1 antigen expression, whereas HNK-1 and, partially, CD16 were lost. Finally the cytolytic activity of LAK effector cells generated from PBMC cultured for 3 d in rIL-2 was susceptible to inhibition by the anti-LAK-1 mAb.