Antihuman lymphocyte rabbit (or horse) gamma globulins used in recipients of organ transplantation are prepared against thymocytes or immortalized cell lines, the only two sources so far allowing enough antigen preparation. These cells lack, however, the surface determinants characteristic of alloreactive blasts involved in the rejection process. We have derived long-term cultures of a panel of alloreactive (untransformed) clones from a rejected kidney. Among them, clone 1E7 has been chosen as a cytotoxic CD4+(CD2+CD3+TCR alpha beta+) clone proliferating against HLA-DR8 targets. This clone (clonality assessed on T cell receptor genomic rearrangements) has been grown using weekly stimulations with the kidney donor—derived EBV cell line and recombinant IL-2. Clone cultures have been adapted to mass production after optimization of culture conditions satisfying pharmaceutical requirements. This procedure warranted a reproducible source of antigen since the functional and phenotypic characteristics of the immunizing 1E7 cells remained identical through the life span of the culture. In addition the study of the total growth capacity of 1E7 cells showed consistent expansion until the 40th cell cycle, ensuring a progency that will satisfy the large-scale requirement for a clinical trial. Rabbits were injected with 100.1081e7 cells (21, 14, and 7 days before bleeding). Sera were depleted of agglutinin by red blood cell absorption and globulin antiblast (GAB) prepared by SO4Na precipitation and ion exchange chromatgraphy; 50% complement-mediated target cell lysis and 50% inhibition of E rosette formation and alloproliferation were obtained at GAB could significantly prolong skin grafts when given prophylactically. Finally GAB have been used in human recipients of kidney grafts for prophylaxis of early rejection. Results of this pilog study are given in a separate report in this issue. In conclusion we have, for the first time, set up a large-scale preparation of polyclonal globulin against a normal human alloreactive clone, and this new reagent should present several advantages over classic antilymphocyte or antithymocyte sera because it contains specificities against activation antigns and has less crossreactivity variation among batches.