The important sources of allergens in cockroach were investigated. Aqueous extracts were prepared from American and German cockroach whole body, cast skins, and feces as well as egg shells of German cockroach. Each extract was analyzed for protein content and characterized by electrophoretic analyses. Cockroach specific IgE in the sera of patients and controls was investigated by RAST, by use of paper discs coupled to equal amounts of protein from each extract. The results for German cockroach indicate that whole body and cast skins have equal RAST potencies, egg shells are less potent, and feces are least potent. Reactivity of the same individuals to American cockroach was much lower. RAST-inhibition studies with German cockroach extracts indicate that the same allergens are found in whole body, cast skins, egg shells, and feces. The RAST potencies obtained in the inhibition experiments, in general, agreed with the direct RAST results. The results indicate that cast skins and whole bodies of German cockroaches contain the clinically relevant allergens for this group of cockroach-sensitive patients.