The heterogeneity of crystalline rhodanese from bovine liver has been studied. The enzyme was separated by preparative disc electrophoresis into four stable forms designated I(3.5%), II(17.0%), III(62.0%) and IV(17.5%). All these components have the same molecular weight of about 35,000 both in native and denaturing conditions. No differences in the primary structure have been detected in amino acid composition and by peptide mapping. Although the isolated forms show similar values of specific activity, except form I, their content of transferable sulfur is different. The enzyme-sulfur complexes of the four components have slightly different circular dichroism spectra in the near ultraviolet, though their spectra in the sulfur-free state are identical. We conclude that these forms are conformational isomers originated from form IV which is predominant in the mitochondrial extract of liver cells.