An unstable hemoglobin was detected by isopropanol and heat precipitation tests in a 49-year-old Japanese man suffering from acute exacerbation of a chronic hemolytic disorder which was apparently triggered by infection of cholelithiasis. One of his two sons carried the same abnormal hemoglobin, and was jaundiced, but otherwise healthy, without anemia. The abnormal hemoglobin focused at a slightly more anodic position than Hb A in thin layer polyacrylamide gel electrofocusing. The abnormal beta chain emerged after normal beta chain in reverse phase high performance liquid chromatography of the hemolysate. It comprised 16.7% and 25.5% of the total beta chain in the propositus and his son, respectively. The partially heme-depleted abnormal beta subunit was precipitated with p-chloromercuribenzoic acid, and the abnormal beta chain was isolated by urea CM-cellulose column chromatography. Structural analysis demonstrated substitution of proline for histidine at position 97 (FG4) in the beta chain. The abnormal hemoglobin was purified by ion-exchange column chromatography. It showed a hyperbolic oxygen equilibrium curve indicating a high oxygen affinity and the absence of cooperative intersubunit interaction. Subunit dissociation seemed to be slightly enhanced. The variant was markedly susceptible to oxidation and rapidly lost heme upon oxidation.