The enzyme amylase was shown to be present in extracts prepared from both house dust and spent growth medium used in the culture of the mite Dermatophagoides pteronyssinus. In dust, it was shown to correlate with both mite counts and concentrations of the faecally derived mite allergen, Der p I. Mite amylase was isolated from the culture medium and shown to be a single chain protein with a molecular weight of 56,000. The enzyme contained free sulphydryl groups and had the N-terminal sequence, KYXNPHFIGXRSVITXLME. It was found to be an allergen using sera from adults (46% positive) and children (25%) who were mite allergic. The expression of allergenicity was dependent on the integrity of intra-chain disulphide bonds.