Assays with different specificity are used for cyclosporin monitoring in clinical transplantation. A recent survey of 35 centers showed that 86% used immunoassays for cyclosporin A (CsA). In consensus documents the following performance criteria were recommended: (a) imprecision < or = 10% at 50 microg/L and < or = 5% at 300 microg/L; and (b) comparison with the reference method (HPLC) should yield a slope of 0.9-1.1, an intercept of -15 to 15 microg/L, and S(y/x) < or = 15 microg/L. The newly developed CsA assays for the AxSYM (Abbott) and the CEDIA (Boehringer Mannheim) as well as the Emit assay (Behring Diagnostica) were evaluated. Results from samples of heart, kidney, and liver recipients (100 specimens each) were compared with a validated HPLC-ultraviolet detection method. Between-series imprecision (CV) with commercial controls was 5.8% and 1.7% for AxSYM (70 and 300 microg/L), 11% and 5.5% for CEDIA (90 and 200 microg/L), and 8.1% and 4.5% for Emit (63 and 172 microg/L). In the presence of 300 microg/L parent CsA, cross-reactivities were (for AxSYM, CEDIA, and Emit, respectively) 7%, 4%, and none for AM1 (1 mg/L) and 12.6%, 25%, and 6% for AM9 (0.5 mg/L). Comparison with HPLC showed in heart and kidney recipients an average overestimation with the Emit and the CEDIA of approximately 22%, with overestimation in the AxSYM of 32%. In liver recipients, the most challenging patient group, the CEDIA and the AxSYM showed a mean overestimation of 43% and 47%, respectively, and the Emit differed by 31% compared with HPLC. None of the immunoassays fully satisfied the performance criteria recommended in the consensus documents. In terms of specificity, Emit ranks before CEDIA, which ranks before AxSYM. Regarding imprecision, the ranking is AxSYM < Emit < CEDIA. These limitations must be considered when using these assays for therapeutic drug monitoring of CsA in clinical transplantation.