In HALG-treated dogs with cardiac allografts a suppression of anti-HALG antibodies was obtained by previously rendering the dogs tolerant to HIgG. The cardiac allografts of HIgG-tolerant and HALG-treated dogs, which had no anti-HALG antibodies in the antigen-elimination test, survived significantly longer (mean survival time: 8.2 ± 1.0 days) than transplants of HIgG-sensitized dogs which had been sensitized 6 weeks prior to transplantation (mean survival time: 5.2 ± 1.0 days). This indicates that the immunosuppressive effect of HALG (as tested in this system) is abolished by the presence of autogenous ‘anti-HALG antibodi