The HNK-1(Leu-7) monoclonal antibody selectively identifies a population of human granular lymphocytes with natural killer (NK) cell activity. We previously reported that the HNK-1+ cell fraction purified from blood mononuclear cells accounted for virtually all NK activity in six individuals. In this study we analysed additional normal individuals and found that in eight out of 14 donors HNK-1+ cells, purified with a fluorescence-activated cell sorter (FACS), exhibited greatly enriched NK cell activity, whereas HNK-1- cells did not have significant activity. In four donors the HNK-1+ cells were enriched in NK activity compared with HNK-1- cells; however, the HNK-1- cells also had moderate levels of activity. In the two remaining donors, NK activity was not enriched in the HNK-1+ fraction in comparison with the HNK-1- fraction. To determine the cell type responsible for NK activity in the HNK-1- subset, these cells were further sorted with the FACS onto OKM1+ and OMK1- fractions and analysed for morphology and function. HNK-1- OKM1- cells were found to be small- to medium-sized lymphocytes devoid of NK activity in all donors tested, whereas most HNK-1- OKM1+ cells were granular lymphocytes and in some donors demonstrated NK function at a level comparable to HNK-1+ cells. Thus some individuals have an important subset of granular lymphocytes with NK-cell activity and the HNK-1- OKM1+ phenotype. It is important to account for these cells in studies involving granular lymphocytes and NK cell function.