The allergen Dpt 4 from the house dust mite Dermatophagoides pteronyssinus was shown to be a lipoprotein as judged by its Sudan black B staining properties. Ultracentrifugal studies confirmed this finding and showed that Dpt 4 was polydisperse, although the majority of the allergen belonged to a very high density lipoprotein class. Physicochemical studies using gradient polyacrylamide gel electrophoresis showed that Dpt 4 had an apparent molecular weight of 244,000 daltons and a particle diameter of 10.73 nm. Agarose gel electrophoresis showed that the allergen had an electrophoretic mobility 0.86 times that of human alpha-lipoprotein. The observation that Dpt 4 is a lipoprotein is consistent with earlier studies which showed that the allergen was heterogeneous with regard to its molecular weight and that it contained the hapten phosphorylcholine.