The risk of complications of immunosuppressive treatment in organ transplantation increases with the aggregate amount of immunosuppressive medication given to the patient. As the doses of immunosuppressive agents required to achieve comparable effects show considerable variability, methods to assess individual sensitivity toward immunosuppressive regimens are urgently needed. The aim of this study was to develop such an in vitro test system. As immunological model for allogeneic transplantation, individual pairs of recipient-derived lymphocytes and of donor-derived B lymphocytes mimicking HLA expression of cells in the transplanted organ were isolated and assessed in mixed-lymphocyte cultures (MLC). Alloreactivity was readily observed and MLC consisted of CD8(+) and CD4(+) T cells as well as CD56(+) natural killer cells. A proliferation assay to measure the response of individual MLC on the immunosuppression by cyclosporine (CsA) was developed. The concentrations of CsA leading to growth reductions by 50% (inhibitory concentration 50, IC(50)) were found between 110 and 220 ng/mL, which was near the trough whole blood levels for CsA. Accordingly, the IC(90) values (660 to 1760 ng/mL) were near the target values for peak whole blood levels. We believe that these data present a simple and potentially useful in vitro technology that allows for the prediction of individual responses to immunosuppressive therapeutic regimens.