Abstract 1. 1. The amino acid and carbohydrate composition of α- and β-hemocyanin of Helix pomatia were determined. The amino acid composition is strikingly similar in both hemocyanins; the carbohydrate composition shows small quantitative differences. The presence of an unidentified sugar in both α- and β-hemocyanin is observed. 2. 2. Both α- and β-hemocyanin have N-terminal arginine; the C-terminal amino acid could not be identified by several methods. By application of a quantitative dinitrophenylation micromethod it was shown that 1 mole of arginine is present per 25 000 g of hemocyanin. 3. 3. Dissociation of hemocyanin molecules beyond the state of twentieth molecules is extremely difficult. Application of 70% formic acid leads to dissociation into subunits with a molecular weight of 25 000, readily dimerizing to 50 000. 4. 4. The obtained subunits are highly heterogeneous, in accordance with the results of dissociation experiments. The cause of this phenomenon is still unknown.