Three new monoclonal antibodies (MAbs) termed 7A6, PP35 and A6/143 were isolated after mouse immunization with CD3- CD16+ NK clones. The screening procedure was based on the ability of MAbs to trigger cytolytic activity of the immunizing clones in a re-directed killing assay against the P815 murine mastocytoma cell line. The 7A6 MAb reacts with 58 kDa surface molecules that appear to belong to the same molecular family defined by the previously described NK-sub-set-specific GL183 and EB6 MAbs. However, unlike from these MAbs, the 7A6 MAb reacted with (and activated) all CD3- NK lymphocytes, independent of their sub-set assignment (based on the expression or lack of expression of EB6, GL183 and CD16). The PP35 MAb reacted with a 70 kDa surface molecule expressed on all CD3- NK cells, as well as on TCR gamma/delta + cells and on a small sub-set of TCR alpha/beta + CD8+ lymphocytes. The PP35 MAb induced activation of essentially all NK cells, although clonal analysis revealed quantitative differences in the magnitude of the cytolytic responses elicited in different clones. Finally, the A6/143 MAb reacted with a molecule of 115 kDa expressed by all human PBL. Similarly to 7A6 and PP35 MAbs, the A6/143 MAb activated all sub-sets of cloned NK cells.