Serial determination of soluble CD8 (sCD8), soluble IL-2 receptors (sIL-2R), and tumor necrosis factor-alpha serum levels were performed in bone marrow transplant patients upon initiation, day 0 (D0) and at D10 of an anti-IL-2 receptor (alpha chain) monoclonal antibody (B-B10) in vivo treatment for steroid-resistant grade greater than or equal to 2 acute graft-versus-host disease (aGVHD). D0 and D10 sCD8 serum levels correlated strongly with response to B-B10 treatment (p = .003 and .001, respectively); 76% of the patients with D0 sCD8 levels less than 500 U/ml responded favorably to B-B10 treatment, versus only a 30% response if the sCD8 levels were greater than 500 U/ml (p = .02). Likewise, D0 tumor necrosis factor-alpha levels significantly correlated with subsequent response to B-B10 treatment (p = .03). D0 sIL-2R levels were not significantly different in B-B10-responsive and nonresponsive aGVHD patients. These results suggest that the serial determination of sCD8 and TNF serum levels could provide valuable predictive information as to steroid-resistant aGVHD responsiveness to anti-IL-2R treatment.