Polyclonal antithymocyte globulin (ATG)/antilymphocyte and antilymphoblast globulins (ALG) antibodies have been used successfully in transplantation, aplastic anemia and graft-versus-host disease. Flow cytometry has been used to analyze peripheral blood lymphocyte populations in transplant patients receiving polyclonal ATG/ALG preparations for immunosuppression. Recent reports have indicated clinical dose adjustment based on levels of patient's cells expressing various CD antigens. In vitro analysis of individual polyclonal ATG/ALG CD antigen specificity could identify appropriate antigens for clinical monitoring as well as provide useful in vitro activity data. Therefore, a flow cytometry based assay to characterize and compare activities to specific CD antigens found on the surface of peripheral blood lymphocytes has been developed. Activities found in four lots each of horse ATG (ATGAM, Upjohn), rabbit and horse ATG (thymoglobulin and lymphoglobulin, Merieux), horse ALG (Minnesota), and rabbit ATG (Fresenius) have been compared for CD2, CD3, CD4, CD5, CD7, CD8, CD11a, CD18, CD28, CD44, CD45, and TCR-alpha/beta antigens. Quantitation is achieved by measuring the concentration of ATG/ALG required to give 50% inhibition of antigen specific fluorescent-labeled monoclonal antibody relative to buffer controls. The three horse products tested have similar activity to most antigens tested. However, Fresenius rabbit ATG has the lowest activity for almost all antigens tested whereas the Merieux rabbit ATG has activities closer to the horse products. This technique allows for rapid in vitro comparison of reactivities to individual lymphocyte antigens as well as in vitro analysis of consistency.