Monoclonal antibodies are being used as specific therapeutic agents for treating organ rejection. This type of treatment requires that the pattern and type of infiltrating cells in the graft during acute rejection be defined. In this study 24 endomyocardial biopsies from cyclosporine-treated human heart allograft recipients were obtained during acute rejection. The biopsies were stained with immunological markers to define the infiltrating cells. The biopsies were divided into three groups; mild, moderate and resolving acute rejection. In all groups T-lymphocytes (marked with the monoclonal Leu-4) were the predominant cells with absence of B-lymphocytes (marked with T015). The results of this study showed that cyclosporine-treated heart transplant recipients have a mixed suppressor/cytotoxic (Leu-2a) and helper/inducer (Leu-3) myocardial infiltrate in contrast to the biopsies of recipients treated with azathioprine, in which suppressor/cytotoxic T-lymphocytes (Leu-2a) were predominant. A clear sequential pattern of changes in the T-cell subpopulations did not emerge, although macrophages became more prevalent with resolving rejection.