It is widely believed that calcium antagonists such as diltiazem exert immunosuppressive effects in kidney graft recipients--however, the mechanism is unclear. In a randomized controlled trial, kidney graft recipients who received diltiazem during transplantation and for an average of 12 months thereafter experienced significantly fewer rejection episodes than patients treated with cyclosporine and steroids alone. Furthermore, 1-year (97% vs. 85%) and 4-year (80% vs. 70%) graft survival rates were higher in diltiazem-treated patients, but the difference was not statistically significant. In vitro, diltiazem had little immunosuppressive activity. Concentrations of diltiazem which blocked the proliferation of PHA-stimulated human peripheral blood mononuclear cells, or prevented activation-associated accumulation of interleukin-2 mRNA, or p50- and p70-IL-2 receptor mRNA exceeded pharmacological concentrations by more than 100-fold. Both, CsA and high doses of diltiazem caused an increase of IL-6 mRNA. In contrast to these findings, the IL-6 plasma concentrations were comparable in both groups, whereas the serum concentration of soluble IL-2 receptors was decreased in patients treated with diltiazem. Administration of diltiazem caused an alteration of CsA metabolism. The whole-blood concentration of CsA metabolite 17 was significantly increased in diltiazem-treated patients, resulting in a five-times-higher concentration of this metabolite in the cellular blood compartment compared with the parent drug. Changes in metabolites 1, 8, and 18 levels were less pronounced. Although direct immunosuppressive properties of diltiazem are unlikely, diltiazem could support immunosuppression by altering CsA metabolism, and promoting accumulation of certain metabolites.