Upon the basis of the results obtained hitherto in the studies of the stereochemical reaction mechanisms, the author has concluded that the stereospecificity of enzymes has essentially a relative character. And the author has pointed out the possibility of the occurrence of a new enzyme "inverskinase", which catalyses the reversible inversions such as D-compound ${leftrighyarrow}$ L-compound in living organisms, and considered the stereospecificity of enzymes and the optical purity of components of living organism from this new standpoint.