Regenerating protoplasts of the basidiomycete Schizophyllum commune were labelled with radioactive N-acetylglucosamine. The label was mainly incorporated in the alkali-insoluble wall fractions R-glucan (containing some hexosamine) and chitin. Light microscopic autoradiography showed a uniformly dispersed incorporation of label in primary cells and chaincells but emerged hyphae revealed a highly localized incorporation at the hyphal apex. Electron microscopic autoradiography showed that the label mainly appeared just outside the plasmalemma in the wall suggesting that chitin and R-glucan are synthesized at the plasmalemma/cell wall interface.