We have evaluated the effect of in vivo administration of hypoxanthine derivative ST 789 on the reconstitution of natural killer (NK) cell activity of mice undergoing syngeneic bone marrow (BM) transplantation. Lethally irradiated BD2F1 mice were injected i.v. with 10(7) syngeneic BM cells and treated with ST 789 (50 mg/Kg ip) from day 1 through day 3 after BM graft. The splenic NK activity was evaluated at 7, 10, 14 and 17 days after irradiation and BM graft by the in vitro cytotoxicity assay against NK-sensitive YAC-1 target cells. Results indicate that treatment with ST 789 significantly enhances the NK activity reconstitution as compared to medium treated controls. These results suggest that ST 789 treatment is capable to induce the in vivo expansion of BM NK-progenitor cells resulting in a shorter time of regeneration of NK cell activity.