In a study of 46 strains of H. pertussis, 34 recently isolated and 12 that had been kept in the laboratory, those precipitated by AIPO4 were also dissolved by ca. 0.2[image] NaOH, dissolved by 10% Na desoxycholate, agglutinated by phase 1 antiserum at dilutions greater than 1/8000, and were virulent for mice. Among the laboratory strains 5 were not precipitated by AIPO4, not dissolved by NaOH or desoxycholate, were only agglutinated by phase 1 antiserum at dilutions less than 1/1000, and were avirulent. Precipitation by AIPO4 appears to provide a quick method for distinguishing virulent and avirulent strains of H. pertussis.