Abstract A homogeneous allergenic protein from cod designated Allergen M was subjected to Edman degradation and found to be blocked at its amino-terminal. By the hydrazine dansyl chloride method the amino-terminal was found to be N-acetylated. Peptide maps of tryptic digested Allergen M yielded 15 ninhydrin-positive spots. The ϵ-amino groups of lysine were masked by ethyl trifluoro thioacetate and maleic anhydride respectively. Hydrolysis by trypsin at the single arginie site of TFA-Allergen M or maleyl-Allergen M, resulted in two polypeptide fragments, which were separated on Sephadex G-50 and designated TM 1 and TM 2. Both fragments were equally active allergenically as assessed by direct skin tests, passive transfer tests and by the radioallergosorbent technique (RAST). Amino acid composition of Allergen M and the fragments TM 1 and TM 2 were determined. Molecular weight estimations by sodium dodecyl sulphate/7·5 per cent polyacrylamide gel electrophoresis revealed 15,000 for Allergen M, 8500 for TM 1 and 6500 for TM 2. Fragment TM 1 was found to be the amino terminal one, with the arginine at its carboxyl-terminal. Allergen M as well as fragment TM 1 contained one residue of glucose. Fragment TM 2 had a freely available alanine as the NH 2-terminal. This fragment was free of carbohydrate as well as sulphur containing amino acids.