A sensitive high performance liquid chromatographic (HPLC) assay was developed to quantitate the organic amine proton-acceptor, tris(hydroxymethyl)aminomethane (THAM) in human plasma, erythrocytes, and whole blood. An aliquot of the sample was heat evaporated (200°C for 2 h), derivatized, extracted, and then injected onto a symmetry C8column. The UV absorbance of the effluent was monitored at 237 nm. 2-Amino-2-methyl-1,3-propanediol was found to be an appropriate internal standard. The method has been applied to quantitate the samples from patients undergoing orthotopic liver transplantation. The assay has also been employed to assess thein vitrotime of THAM-uptake into the erythrocytes from blood samples incubated at 37°C.