['Der p 1 and Der f 1 are highly related allergens from the two main house dust mite species, Dermatophagoides pteronyssinus and Dermatophagoides farinae, respectively. A link between the cysteine proteinase activity of Der p 1 and its allergenicity has recently been demonstrated.', 'To test the effects of several proteinase inhibitors, mainly a chestnut cystatin (CsC), on the enzymatic activity of Der p 1 and Der f 1, as well as to study the potential acaricide properties of the inhibitors.', 'Inhibition tests were performed using mite extracts, as well as isolated Der p 1 and Der f 1 as targets. Immunodetection after SDS-PAGE and N-terminal amino acid sequencing were used to demonstrate the inactivation of chestnut cystatin by Der p 1. Bioassays including different inhibitors in a semisynthetic diet were performed to evaluate a potential effect on larval survival.', 'CsC inhibited the crude digestive proteinase activity of D. farinae, but it was not active towards the equivalent enzyme from D. pteronyssinus. This differential behaviour was fully explained by testing CsC against the two purified allergens, Der f 1 and Der p 1; the former was highly susceptible, whereas the latter was not affected. In contrast, other cysteine proteinase inhibitors, such as egg white cystatin and E-64, inhibited the proteinase activity of both mite allergens. Der p 1 inactivated CsC by a specific proteolytic cleavage between Gly 6 and Val 7, thus given rise to a noninhibitory processed protein. Besides the in vitro activity, CsC also showed an in vivo effect on D. farinae, drastically increasing the larval mortality when added to a semisynthetic diet.'] Are der p 1 and Der f 1 , the highly related and major allergens from house dust mites , differentially affected by a plant cystatin?