The differentiation of keratinocytes involves numerous steps including the formation of the cornified envelope and the aggregation of keratin filaments by filaggrin monomer molecules. In this study, we investigated whether mu-calpain is involved in the processing of profilaggrin to filaggrin monomers by using both an active mu-calpain specific antibody and a 27-mer synthetic calpastatin peptide, a cell-permeable calpain-specific inhibitor. Upon incubation of cultured keratinocytes with Ca2+ for 96 hours, active mu-calpain with a molecular mass of 76 kDa appeared preceded by an increase in mu-calpain mRNA. In synchrony with the appearance of active mu-calpain, the processing of profilaggrin occurred. Furthermore, the Ca2+-induced activation of mu-calpain and the processing of profilaggrin were affected by the addition of the synthetic calpastatin inhibitor. These results indicate that the activation of mu-calpain plays a major role in the profilaggrin processing.