Single nucleotide substitutions are known to result in a different amino acid at one of four sites in cytochrome P4502C9 (CYP2C9) namely: residue 144: Arg/Cys; residue 358: Tyr/Cys; residue 359: Ile/Leu and residue 417: Gly/Asp. Polymerase chain reaction (PCR) - based amplification of the nucleotide fragments encompassing the four residues (144, 358-359 and 417) in 18 samples of human genomic DNA from a liver bank and one sample of DNA extracted from the blood of a known poor metabolizer of tolbutamide has been carried out. The products of PCR amplification were analysed by either allele-specific restriction endonucleases or probed with radioactively labelled allele-specific oligonucleotides in dot blot hybridizations. Fourteen individuals were homozygous for Arg144 and four were heterozygous Arg/Cys144. All individuals analysed were homozygous for Tyr358 (n=17) and for Gly417 (n=18). With the exception of one heterozygote the other 17 subjects were homozygous for IIe359. The genotype of the known poor metabolizer of tolbutamide was homozygous for Arg144, Leu359 and Gly417. The relative levels of expression of the Cys and Arg144 alleles was studied in the heterozygotes. A relative 5- to 10-fold greater expression of the Cys- over the Arg144 allele was noted in two heterozygotes. There was no apparent correlation of genotype to the hydroxylation of the known CYP2C9 substrates phenytoin, tolbutamide, torasemide and diclofenac. Apparent Kmvalues for the cDNA-expressed Arg144/Ile359, Cys144/ Ile359 and Arg144/Leu359 variants towards tolbutamide were 91µM, 62µM and 229µM, respectively. It is likely that functional changes occurring as a result of the Ile359Leu transition are responsible for the tolbutamide poor metabolizer phenotype.