The FCL provides support for clinical trials of FK 506 in liver transplantation by monitoring drug concentrations in plasma and whole blood. The sensitive EIA method of Tamura et al was adapted for routine use. Extraction of drug from 300 microL plasma or 20 microL whole blood samples was carried out using MeCl. An alternative solid phase (Sep-Pak) extraction method was also tested. Nonlinear temperature-sensitive binding of FK 506 to red cells necessitates plasma separation from whole blood at 37 degrees C. The analytical procedure entails numerous steps and efforts were made to improve the accuracy of the method. Interday CVs of FK 506 in plasma (0.3 to 3.0 ng/mL) and whole blood (4 to 60 ng/mL) ranged up to about 20% during validation testing and are maintained below 30% during routine assay use. However, daily QC samples occasionally yield unexpectedly high FK 506 concentrations. The MeCl and solid phase extraction methods yield FK 506 concentrations that reasonably correlate but the MeCl method produces plasma concentrations that are about 30% lower. FK 506 is appreciably bound in red cells with a nonlinear WBPR of 20 to 50 at low plasma concentrations (0 to 2 ng/mL) and a ratio of about 11 at plasma concentrations above 5 ng/mL. This may complicate conversion from plasma to whole blood for routine therapeutic monitoring. The described procedures for FK 506 are being implemented at various clinical sites in the United States with the FCL providing assistance with a quality assurance program to assure intersite comparability of assays results.