The physicochemical and immunochemical characteristics of the allergens from Dermatophagoides pteroyssinus have been described, in particular those for the allergens designated Dpt 4 and Dpt 12. Dpt 4 is a high molecular weight allergen with an apparent molecular weight of 274,000 daltons and a pI of 4.5. In contrast, the antigenically unrelated Dpt 12 has an apparent molecular weight of 24,000 daltons and a pI 6.6. Both allergens demonstrate heterogeneity, either in molecular weight or pI. Dpt 12 appears to be the major allergen in extracts of D. pteronyssinus and probably corresponds to the low molecular weight allergen previously described by other workers.