A method for genotyping poor metabolisers of debrisoquine is based on specific polymerase chain reaction (PCR) amplification of parts of mutant genes for hepatic cytochrome P450IID6. Analysis by restriction fragment length polymorphism allowed identification of only 25% of poor metabolisers, but when it was combined with allele-specific PCR over 95% of poor metabolisers could be identified. The PCR method also allowed the identification of heterozygous carriers of mutant alleles.