The proteins precipitated with ammonium sulfate from the urine of a patient (Mat) with multiple myeloma were separated into three components by ion-exchange and gel chromatographies. Sodium dodecyl sulfate polyacrylamide gel electrophoresis, amino acid analyses, immunochemical tests, and measurement of circular dichroism showed that these components were a dimer with a disulfide bond, a stable monomer, and a variable fragment, respectively. All three protein components reacted with 5,5'-dithiobis-(2-nitrobenzoic acid) in Tris-HCl buffer at pH 8.0, indicating that they contained free sulhydryl groups. Partial reduction with dithiothreitol in the absence of denaturants yielded two SH groups per molecule from both the monomer and the dimer, and one SH group per molecule from the fragment. This indicates that the monomer of Mat protein contains a cysteinyl residue in the variable region in addition to a cysteinyl residue at the COOH terminus. The reactivities of the two SH groups of the partially reduced monomer toward iodoacetamide and iodoacetic acid were studied by polyacrylamide gel electrophoresis. The two SH groups had similar reactivities with iodacetamide, but the SH group at the COOH terminus was more reactive with iodoacetic acid than that in the variable region. The extrinsic Cotton effects of an azobenzene-2-sulfenyl group introduced into the SH group in the variable region were different from those of dye attached to the COOH terminal SH group, indicating that the two SH groups had different environments. The states of the SH groups of the intact monomer are discussed on the basis of these findings.