OBJECTIVE To explore the molecular mechanism for a blood sample with mixed-field hemagglutination upon determination of ABO blood group. METHODS Serological techniques were employed to identify the erythrocyte phenotype. The A and B antigens were detected by flow cytometry. The preliminary genotype of ABO gene was assayed with sequence-specific primer-polymerase chain reaction (PCR-SSP). Exons 6 and 7 of the ABO gene were amplified with PCR and analyzed by direct sequencing. Haplotypes of the ABO gene were analyzed by cloning sequencing as well. RESULTS The serological reaction pattern has supported an O phenotype when all the tubes were centrifuged for the first time. However, a mixed-field hemagglutination of red blood cells (RBCs) with anti-A antibodies was present after the tube was centrifuged five times later. A antigens were detected on the surface of partial red blood cells of the sample by flow cytometry. PCR- SSP results have shown that the preliminary ABO genotype was A/O. Analysis of the fragments of exons 6 and 7 of the ABO gene has indicated that heterozygosis lied as follows: 261G/A, 425T/T, 467C/T, 646A/T, 681A/G, 745C/T, 771C/T, 829A/G, conjecturing the genotype to be A307/O02, which was confirmed by haplotype sequence analysis. Compared with A101 allele, A307 allele has two missense mutations, 467C> T and 745C> T, which have resulted in substitutions Pro156Leu and Arg249Trp in the A glycosyltransferase polypeptide chain. CONCLUSION A variant allele (A307) has been identified for the first time in mainland China, which is responsible for the formation of A3 phenotype.