1. We have studied band 3 HT, a human red-cell band 3 variant with increased M(r), which is associated with abnormal red-cell shape (acanthocytosis) and increased anion-transport activity. 2. We have shown that the increased M(r) does not result from the presence of the band 3 Memphis mutation, and that the variant band 3 is covalently labelled by 4,4'-di-isothiocyanato-1,2-diphenylethane-2,2'-disulphonic acid (H2DIDS) less readily than normal. 3. cDNA cloning studies show that band 3 HT results from the mutation Pro-868-->Leu, and the possible significance of the mutation in the altered anion-transport activity and cytoskeleton binding properties of band 3 HT is discussed.