SerpinB6 (Proteinase inhibitor 6/PI-6) is an intracellular serpin produced by leukocytes, platelets, endothelial cells, keratinocytes and other epithelial cells. It is a potent cathepsin G inhibitor thought to protect monocytes, neutrophils and bystander cells from ectopic cathepsin G during inflammation. Here we show that serpinB6 also inhibits the human serine protease kallikrein-8 (hK8) and that in human and mouse skin, serpinB6 and kallikrein-8 co-localize in differentiated keratinocytes. SerpinB6 inhibits hK8 with an association rate constant (kass) of 1.8 +/- 0.2 x 10(5) M(-1)s(-1) compared to 3.4 +/- 0.2 x 10(6) M(-1) s(-1) for the interaction between the mouse orthologue of serpinB6 (SPI3/serpinb6a) and mouse kallikrein-8 (mK8). Molecular modelling suggested that the lower efficiency of the serpinB6/hK8 interaction is partly due to the bulkier P2 methionine residue of serpinB6 compared to the smaller P2 valine in SPI3. Taken together, these results suggest that serpinB6 is a physiologically relevant inhibitor of hK8 in skin. We postulate that serpinB6 protects the intracellular compartment of keratinocytes from ectopic hK8.