An extract of the cockroach Blattaria sudamericana (BS), a very common insect in Argentina, was passed through Sephadex G-200 and DEAE cellulose ion exchange columns. This method yielded 2 proteins and 6 hexoses whose quantities were determined by absorbance in a LKB Uvicord spectrophotometer at 280 nm and 470 nm respectively. Protein contents were also recorded by the Lowry method and the micro-Kjehldal technique. Adult albino rabbits were immunized with a mixture of BS and complete Freund's adjuvant over a period of thirteen weeks. The antisera obtained were studied by the Ouchterlony method, the immunoelectrophoresis and the passive hemagglutination technique against BS and the eluated fractions with the highest protein content. Molecular weights were studied with the usual marker proteins subjected to gel filtration by a Sephadex G-200 column. Each protein concentration was 13.5 mg/ml in a volume of 1.5 ml meanwhile BS was 42 mg/ml. This antigen showed a molecular weight of 180,000 daltons. Fifty untreated atopic patients of both sexes with ages between 19 and 74 years suffering perennial rhinitis and bronchial asthma were chosen according to the American Thoracic Society criteria. Their IgE levels ranged between 110 and 3000 KU/L with an average of 620 KU/L. IgE anti-BS-RAST showed positive results in half of the patients while type I positive skin tests with BS 1:100 appeared in 62% of the atopic group.