Isolated nucleic acids encoding an allergen of Dermatophagoides pteronyssinus, Der p III, are disclosed. A cDNA encoding a peptide having a Der p III activity and a predicted molecular weight of about 24,985 daltons is also described. The nucleic acids can be used as probes to detect the presence of Der p III nucleic acid in a sample or for the recombinant production of peptides having an activity of Der p III. Peptides having an activity of Der p III can be used in compositions suitable for pharmaceutical administration or methods of diagnosing sensitivity to house dust mites.