A method for the analysis of thiopurine nucleotides in human transbronchial lung biopsy was developed. The sample treatment procedure is based on perchloric acid homogenisation and deproteinisation with dithiothreitol and hydrolysis of thiopurine nucleotides into their free bases by heating of the acid extract. Then, the free bases were analyzed in the gradient elution mode on a Hypersil ODS, 3-microns column using dihydrogenphosphate buffer-methanol as eluent. Mean analytical recoveries for 6-thioguanosine monophosphate and 6-thioinosinic acid from lung tissue were 97.0 +/- 2.0 and 98.0 +/- 1.8% at a concentration of 3.0 nmol/ml and the minimum detectable amounts were 3.5 and 2 pmol, respectively. The procedure described is simple and represents a suitable method for the investigation of thiopurine nucleotides in tissues.