Folding of cathepsin S, like other cathepsin L-like proteases, depends on its proregion. The major part of the proregion forms a small domain distal from the catalytic centre, suggesting function(s) beyond active-site shielding. Using an optimised in vitro trans-refolding assay, we compared reactivation of denatured cathepsin S by the genuine propeptide, wild-type and ten selected mutants. Including structural data and binding constants, we identified the prodomain core and the hairpin region to be important for the foldase function.