The presence of a high molecular weight glycoprotein component bearing the hapten phosphorylcholine (Pc) has been demonstrated in some but not all extracts of house dust mite using a phosphorylcholine specific IgA myeloma protein (S107). The Pc bearing component was isolated from house dust mite extracts by precipitation at equivalence using whole myeloma serum and the isolated fraction was found to be allergenic as judged by a direct RAST assay using a highly house dust mite allergic human serum pool. RAST inhibition studies using free hapten and direct RAST studies using pneumococcal polysaccharide C-substance indicated that the Pe moiety did not contribute significantly to the immunodominant portion of the isolated allergen. Further studies indicated that the S107 isolated material was immunochemically different from the tridacnin reactive material and the con A reactive material previously isolated fom extracts of house dust mite.