Therapy with antilymphocyte globulin (ALG) has been shown to be effective in restoring hematopoiesis to some patients with aplastic anemia. It would be useful to have a method for predicting those likely to be responders versus nonresponders. The mode of immunostimulatory action of ALG is of interest in addition to its immunosuppressive action. We examined in vitro the distribution of the proliferative responses of ALG-stimulated peripheral blood mononuclear cells (PBMCs) obtained from 18 patients with aplastic anemia, eight of whom responded to ALG and 10 who did not. We found a significant difference in the proliferative response of PBMCs obtained from the eight responders versus the 10 nonresponders (P less than .01). Two-color flow cytometry analysis of the patients' PBMCs stimulated by ALG in vitro showed that the CD4-positive subsets were activated to a greater extent by ALG than the CD8-positive subsets. Moreover, a positive correlation with the clinical response of patients to ALG with granulocyte-macrophage colony-stimulating factor produced by their PBMCs stimulated by ALG suggests that the immunostimulatory property of ALG has an important role in the treatment of aplastic anemia. Our results suggest that the clinical response to ALG therapy is correlated with its lymphocyte proliferative effect in vitro, and indicates that the assessment of the proliferative response of PBMCs in vitro would be useful in predicting the clinical response to ALG therapy.