From September through November of 1985, a mixed outbreak of Bordetella pertussis and Bordetella parapertussis occurred in an apartment house in Ashikaga City, Japan. Bacteriological and serological investigations were conducted on seven families, comprising 13 adults and 19 children, living in the same apartment house and having social contacts. For all the subjects, cultivation of nasopharyngeal mucus and measurement of pertussis agglutinin antibody, anti-PT antibody, and anti-F-HA antibody were performed. On the basis of the clinical, bacteriological or serological findings, seven children of three families (families A, B and C), including the first child patient (index case) of family A, were diagnosed as having infections caused by B. pertussis, or B. parapertussis or both. B. pertussis and/or B. parapertussis were isolated from six of the seven child patients. B. pertussis alone was isolated from three children (one in each family), and two of them had not been vaccinated. B. parapertussis alone was isolated from two children, while both bacteria were isolated from one child. Those three children from whom B. parapertussis was isolated were in family C, which was the last of the three families to develop symptoms of infection. Josamycin had been administered to two of the three children from whom B. parapertussis was isolated before the nasopharyngeal mucus specimen was collected. In one of the two children from whom only B. parapertussis was isolated, the significant elevation of anti-PT antibody titer was observed, which was indicative of a mixed infection with B. pertussis.(ABSTRACT TRUNCATED AT 250 WORDS)