The characterization of cod somatolactin (SL), a new pituitary protein belonging to the growth hormone/prolactin family, is described. Cod SL has a molecular weight of 26 kDa and consists of 209 amino acids, of which eight are Cys. The protein has three disulfide bonds between residues Cys5-Cys15, Cys65-Cys181, and Cys198-Cys206. The Cys residues at positions 42 and 180 are not involved in disulfide bonding. The positions of these disulfide bonds are homologous to those found in prolactin and growth hormone. Cod SL has two possible N-glycosylation sites, but only one appears to have carbohydrate units attached. Chemical analysis showed the following sugars to be present: galactose, mannose, N-acetylneuramic acid, and glucosamine. A smaller variant (23 kDa) of SL has been isolated, which is believed to be deglycosylated. Sequence comparison revealed cod SL to be similarly related to both GH and PRL, but slightly higher identity was observed to the tetrapod hormones (27-33%) than to the teleost hormones (21-27%).