Bovine liver thiosulfate sulfurtransferase (rhodanese) (EC 22.214.171.124) has been reported to exist in solution in a rapid, pH-dependent equilibrium between monomeric and dimeric forms of molecular weights 18 500 and 37 000 (Volini, M., DeToma, F. and Westley, J. (1967), J. Biol. Chem. 242, 5220). We have reinvestigated the proposed dissociation using sodium dodecylsulfate-polyacrylamide gel electrophoresis. The smallest rhodanese species observed has a molecular weight around 35 000, which is not reduced by severe denaturing conditions, including alkylation in 8 M guanidine · HCl or dialysis against 2% sodium dodecylsulfate and 5% mercaptoethanol. After limited CNBr cleavage, intermediate products of >18 500 molecular weight are formed. The apparent molecular weight of these intermediate fragments is not changed by addition of mercaptoethanol. The total apparent molecular weights of the CNBr fragments after exhaustive cleavage is approx. 45 000 ± 15 000. These results are not consistent with a monomer molecular weight of approx. 18 500 for thiosulfate sulfurtransferase.