One of the obstacles of BMT that limits its efficacy is failure to eradicate the original tumor. The incidence of tumor relapse is particularly high after autologous BMT. Natural killer (NK) cells comprise various subsets that can express inhibitory receptors for MHC class I determinants. We have recently demonstrated that blockade of NK-cell inhibitory receptors can augment antitumor effects in vitro and in vivo. However, breakdown of tolerance and autoreactivity may occur as a result of the inhibition of NK-cell inactivation to self MHC determinants. We have utilized F(ab')2 fragments of monoclonal antibody, 5E6, against Ly49C/I inhibitory receptors, which are expressed on 35% to 60% of NK cells in H2b strains of mice and are specific for H2Kb, to investigate the effect of inhibitory-receptor blockade on syngeneic bone marrow cell (BMC) and tumor cell growth. We show that treatment of interleukin 2-activated C57BL/6 (B6, H2b) SCID-mouse NK cells with 5E6 F(ab')2 fragments during 48-hour coculture resulted in autoreactivity against syngeneic BMCs as demonstrated by suppression of myeloid reconstitution on day 14 post-BMT. However, this suppressive effect was transient and normalized by day 21 post-BMT. In contrast, blockade of inhibitory receptors during 24-hour coculture had no adverse effects on myeloid reconstitution after BMT. Furthermore, under the same coculture conditions, NK cell-mediated purging of C1498 leukemia cells contaminating syngeneic BMCs was more effective with inhibitory-receptor blockade, leading to a significantly higher proportion of animals with long-term survival compared to the control recipients. These results demonstrate that short-term in vitro blockade of inhibitory receptors can augment antitumor activity without long-term inhibitory effects on BMCs and thus may be of potential use in the purging of contaminating tumor cells prior to autologous BMT.