Pig and horse colipases have been purified by a common procedure using trypsin and carboxypeptidase inhibitors as stabilizers. Two forms of pig colipase were identified : a predominant A1 form with about 103 -105 residues, and a minor slightly degraded A2 form in which the last two C-terminal residues, Asp and Ser, were lacking. This type of degradation is considerably slowed down by carboxypeptidase inhibitors. A total of four forms of the horse cofactor were characterized: two (A1 and Bl) were probably isocolipases which differed by only a few substitutions. Both contained the same number of residues (about 96), an N-terminal valine and an Arg-Ser-Glu-(Glx),,,-Arg C-terminal sequence. A2 and B2 were slightly degraded forms probably resulting from tryptic cleavage of the Arg-Ser bond in the above sequence. The presence of methionine in the horse cofactor allowed fragmentation by cyanogen bromide. The C-terminal fragment was composed of 16 or 17 residues and contained no histidine. The single histidine of horse B1 was found in the intermediary fragment between Met-I8 and Met-(n-16) or Met-(n-17). These data show that the Cterminal parts of both pig and horse colipases are still more exposed to proteolytic degradations than the N-terminal parts. Preliminary attempts to crystallize B1 were carried out.Pancreatic colipase is a small protein which acts as a cofactor for lipase during intraluminar digestion of dietary triglycerides in the presence of bile salts [I -31. It is now generally accepted that lipase alone cannot adsorb in vivo to bile-salt-coated triglyceride interfaces and that it must be anchored by the cofactor [4]. Colipase also protects lipase against interfacial denaturation [5 -71. It is clear that these effects require specific protein-lipid and protein-protein interactions which will only be understood after elucidation of the tridimensional structure of the colipase molecule.