Sublines of the hematopoietic stem cell line K-562 were tested for their susceptibility to human natural killer (NK) cell activity. A correlation was found between the degree of NK-mediated lysis and the presence or absence of particular chromosomal markers. K-562 subline susceptible to lysis by NK were found to express karyotypically a deletion 9- and a marker 8(t1-18), whereas resistant sublines did not express these markers. A cloned K-562 subline B1V was chosen as representative of a resistant subline. This subline was resistant to lysis even after prolonged incubation and activation of the NK cells with interferon. However, it was found that B1V was lysed by both antibody-dependent, complement-mediated and antibody-dependent cellular mechanisms at levels comparable to those seen with NK-sensitive K-562 sublines. Subline B1V did compete poorly with NK-sensitive K-562 in cold-target inhibition; however, conjugate-formation assays demonstrated that the binding of NK cells to B1V cells is comparable to that of NK-sensitive K-562 cells. We suggest that cells of the cloned line B1V are recognized normally by NK cells but do not activate the lytic mechanism of the bound NK cell. Treatment of the resistant clone B1V with neuraminidase did not lead to enhanced levels of lysis. Protein extracts of NK-sensitive K-562 sublines efficiently inhibited lysis by NK cells but extracts of the resistant clone, B1V, did not inhibit lysis, suggesting that the clone lacks cell surface determinants involved in the post-recognitive activation of the NK cytolytic mechanism.