Cell suspensions enriched and depleted for rosette-forming cells with sheep red blood cells (E-RFC) and depleted for RFC with antibody complexes were prepared. The isolated fractions were characterized by cell surface marker analysis and tested for their natural killer (NK) and killer (K) cell activity against K-5 62 cells and IgG-coated P-8 15 cells, growing in suspension, and against a number of monolayer tumor cell lines. It was found that the NK cells most likely belong t o the T cell lymphocyte subpopulation. Furthermore, this study indicates that several subpopulations exist, e.g. NK cells that have no IgG F c receptor (FcR) on their surface and NK cells that bear IgG FcR, indicating that for a proportion of NK cells the IgG FcR is not involved in the NK lytic process and hence antibody-independent. Moreover, monocytes and B lymphocytes appear not t o be directly involved in the NK cell lytic process.Furthermore, cell separation procedures were used t o obtain cell suspensions either bearing IgG FcR or lacking IgG FcR. Cells bearing IgG FcR were isolated in such a way that they lost their IgG FcR by shedding, as a result of the separation procedure. Again, all fractions were simultaneously characterized by cell surface marker analysis and tested for their NK and K cell lytic activity. The effect of immune complexes on the NK and K cell lytic activities was investigated. The data indicate that the IgG FcR is not involved in the NK lytic mechanism, although this receptor may be present o n the NK cell. Moreover, prolonged culturing of lymphocytes increases and/or induces NK cell lytic activity.