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The arcid clam Barbatia reeveana contains an intracellular haemoglobin with an unusual structure. First, compared with other intracellular haemoglobins, it is extremely large, with a mol.wt. of 430000 and an s20,w of 13.6S. A minor component (mol.wt. = 220000; s2O,w = 9.7S) is also present as a probable dissociation product of the major component. Secondly, this haemoglobin has an unusual subunit structure. It contains 1 mol of haem per 16000 g of protein, in common with most other haemoglobins. However, the smallest polypeptide that could be obtained after treatment with sodium dodecyl sulphate or 6 M-guanidine with reducing agent has a mol.wt. of 32000-37000. Digestion of the haemoglobin with the proteinase subtilisin produces both 57000-and 30000-mol.wt. aggregates that contain 1 mol of haem per 16000 g of protein and that can be dissociated into 16 500-mol.wt. polypeptides by treatment with sodium dodecyl sulphate. The intact polymer shows slight co-operativity (h = 1.7), lacks a Bohr effect between pH 7 and 8, and has a low oxygen affinity [P50 = 4.8 kPa (36mmHg) at 200C] relative to other haemoglobins. The 30000-mol.wt. aggregate obtained by digestion of the polymer binds oxygen reversibly with an affinity greater than that of the polymer, but with some co-operativity (h = 1.7). These results are consistent with the hypothesis that the subunits of this unusually large intracellular haemoglobin are 32000-mol.wt. polypeptides that in turn are composed of two covalently linked haem-containing oxygen-binding domains. This is the first report of an intracellular haemoglobin with such a structure.Most intracellular haemoglobins of both vertebrates and invertebrates consist of haem-containing polypeptides with mol

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