Hb. Heathrow [β103(G5)Phe→Leu] was identified in an Englishman with a life-long history of polycythemia, his father had been similarly affected. A hemoglobin variant was suspected from the high oxygen affinity of the patient's blood. The Hb Heathrow abnormalβchain was resolved from the normalβchain by high performance liquid chromatography, and the abnormal peptide and the amino acid replacement were identified by mass spectrometry. The corresponding base change (C→G at codon 103) was demonstrated by sequence analysis of the polymerase chain reaction amplified exon 2 of the genomicβ-globin gene. This is only the third known instance of Hb Heathrow.