Incubation of human plantar stratum corneum (SC) with trypsin at various pH suppressed the typical "keratin pattern" in the cytoplasm of the horny cells and disclosed tonofilaments and desmosomal attachment plates as they are seen in living keratinocyts. Tonofilaments were 62-75 A wide and showed a 100-120 A periodicity. This effect was non-specific since it was also obtained by incubation of SC with citric acid or sodium citrate, but it was not due to hydration alone. Partial acantholysis was obtained by incubation with trypsin followed by citric acid. After simultaneous action of trypsin in sodium citrate and in citric acid, cell membranes were separated from the cytoplasm and many of them were broken around desmosomes with subsequent isolation of desmosomes. The role of trypsin in this effect was uncertain. These findings strongly suggest that the filamentous network built up by living keratinocytes remains almost unchanged during the keratinization process although no longer visible under E.M.