Studies of human polygenic diseases require the genotyping of polymorphic markers from large numbers of subjects. The rapid detection of the insertion (I)/deletion(D) polymorphism of the angiotensin converting enzyme (ACE) gene by the polymerase chain reaction (PCR) has facilitated the study of this locus in a a number of cardiovascular diseases, but not all gene polymorphisms are as easily detected. We describe a rapid mismatch-PCR/restriction fragment length polymorphism (RFLP) assay for the C1166 variant of the angiotensin II type 1 receptor gene, a mutation which may interact with the ACE polymorphism in the determining of risk of myocardial infarction. This rapid assay, which requires no special equipment or expertise, will be useful in confirming the interaction between these two gene loci. The principles utilised can be applied more generally to the detection of any polymorphic single base substitution.