The three-dimensional configuration assumed by a polypeptide chain occurs spontaneously, with or without the participation of molecular chaperones, and is due to the combined interactions of the amino acids in the chain. In the case of disulphide-bond containing proteins, one or more enzymes catalyse the random cleavage and correct reformation of nascent protein disulphide bonds, e.g. protein disulfide-isomerase (PDI). PDI was first discovered in the ninteen sixties and was purified from bovine liver by Anfinsen and coworkers. The PDI active site contains reducible disulphide groups that participate in the reactivation of denatured protein. Generally, its activity is measured with respect to the catalysis of correct disulphide formation, and therefore reactivation, of reduced RNAase. Alternatively oxidized denatured (‘scrambled’) RNAase can be used as substrate and disulphide isomerization leading to reactivation measured. That thioredoxin (TRX), a ubiquitous, monomeric, multifunctional protein may have a role analogous to that of PDI was first suggested because its active site includes an amino acid sequence (Trp-Cys-Gly-Pro-Cys) similar to that in the active site of PDI (Trp-Cys-Gly-His-Cys). These vicinyl Cys residues of TRX and PDI can undergo reversible oxidation to form disulphide bonds. Recently Boniface and Reichert [ l ] pointed out that the Bsubunits of gonadotropic hormones also contain similar vicinyl Cys residues. Bovine, ovine or porcine LH-8 have the sequence His-Cys-Gly-Pro-Cys at positions 89-93 and bovine, ovine, and porcine FSH-8 have a similar sequence (His-Cys-Gly-Lys-Cys) in the same position. They found experimentally that purified ovine FSH and bovine LH preparations were about 60 and 300 times as active as thioredoxin, respectively. We used the assay of Pigiet et al. [2] with reduced, denatured RNAase as substrate to compare the activities of gonadotropins @LH and pFSH), PDI and thioredoxin.