The subunits of S100 protein were isolated from surgically resected tissues of human malignant melanoma and schwannoma by means of an affinity chromatography followed by high performance liquid chromatography. The melanoma tissue gave rise to the alpha and beta subunits in almost equal quantities while the schwannoma tissue yielded only the beta subunit, indicating that the S100 subunits were distributed differently between these tumor tissues. This finding suggests that the cellular distribution of S100 subunits varies, so that the cells containing S100 protein can be classified into several types on the basis of the subunit composition.