Electrophoretic analysis of a hemolysate from a young man undergoing a routine physical examination revealed an abnormal hemoglobin with a mobility similar to Hb S on cellulose acetate (pH, 8.4). This new variant, designated Hb Connecticut, was found in three generations of a family of Polish descent. Several individuals possessing the variant exhibited mild anemia. Structural analysis of the abnormal beta-chain indicated that the amino acid substitution was at position 21 (B3), and involved the replacement of aspartic acid with glycine. Oxygen dissociation studies revealed low oxygen affinity. The alkaline Bohr effect and the degree of cooperativity were unchanged. Analysis of the crystal structure of the variant suggested that the low oxygen affinity was due to the possible disruption of salt bridges between aspartic acid 21(B3) and lysines 61(ES) and 65(E9), changes that could lead to steric interference in oxygen binding.