We report the use of a polymerase chain reaction (PCR) format together with denaturing gradient gel electrophoresis (DGGE) which allows rapid identification of the 6 major genotypes (AA, AO, BB, BO, AB and OO) of the human ABO blood group polymorphism in a single amplification. The procedure also distinguishes hitherto undescribed polymorphisms associated with the O and B alleles. Thus in testing 95 unrelated European individuals 4 different O alleles, 2 B alleles and 1 A allele were identified by DGGE and the level of recognisable heterozygosity, and hence the information content of the locus as a genetic marker, was raised from 3/95 (3%) to 66/95 (70%). The procedure is robust, genotyping is rapid and clear-cut, and has immediate implications for the use of the ABO locus in linkage analysis on chromosome 9q, the investigation of disease associations and forensic identification.