The in vivo infusion of CD48 MoAb or CD2 + 48 MoAb in irradiated recipients of syngeneic BM inhibits day 12 CFU-S formation. B6 recipients were irradiated with 7.5 Gy total body irradiation via ¹³⁷Cesium source on day −1 and then administered 10⁵ B6 BM cells on day 0. Mice (n = 8 per group) were injected with CD2 MoAb, CD48 MoAb, CD2 + 48 MoAb, or irrelevant IgG, as indicated on the y-axis at a dose of 300 μg each, administered IP on days -1, 2, 6, and 9 post-BMT. Twelve days post-BMT, mice were killed and spleens were placed into Bouin’s solution to facilitate enumeration of colonies. The mean ± 1 SEM number of colonies is listed on the x-axis. *P < .001 as compared with irrelevant IgG control. ^#Colonies are all smaller in size than observed in irrelevant MoAb–treated controls.