□ A novel β chain variant found in combination with β0-thalassemia (thal) was identified in a male infant by electrospray ionization mass spectrometry (ESI-MS). Analysis of the infant's denatured blood and a 30 min. tryptic digest of his blood identified the mutation as β56(D7)Gly→Cys, which was confirmed by tandem mass spectrometry (MS/MS). We have named this new variant Hb Leeds. The infant's parents, resident in Yorkshire, UK, but originally from Pakistan, were found to have β0-thalassemia (thal) trait (mother) and Hb Leeds trait (father). Hematological data on the infant's parents and siblings are given. Hb Leeds trait was also found in three unrelated Pakistani adults living in the same area of Yorkshire. Hb Leeds trait in adults appears to have few clinical manifestations, but when combined with β0-thal it led to a more severe anemia in the infant than in the corresponding thalassemic trait in his mother.