Bacteriophages from Bordetella pertussis were titrated on the indicator strain B. parapertussis 17903 by using standard soft agar techniques. Secondary growth, occasionally observed in some phage plaques, was isolated and transferred onto selective media. Judging from growth on these special media and microscopic examination the isolated clones consisted entirely of Bordetellae. Determination of the agglutinogen pattern of 160 of these clones revealed that 88% contained agglutinogen 1; 87.5% agglutinogen 14, and 80.1% agglutinogen 12 (Table 3). However, none of the strains expressed the agglutinogen pattern of either the phage donor or the recipient strain. The isolated clones were lysogenic as demonstrated by phage production and immunity against superinfection (95% of the clones).--Absorption of the monospecific antisera with whole cells from lysogenic strains resulted in a drastic decrease or even complete loss of specific antibodies towards those antigens identified by slide agglutination reactions (Table 4). Cross absorption experiments with antisera against B. pertussis, B. parapertussis and strain 73 1/7 and various strains of the genus Bordetella and with a number of lysogenic strains showing various agglutination patterns allowed the conclusion that the latter ones were serologically related to B. pertussis. The lysogenic strains completely absorbed antibodies against B. bronchiseptica, and those strains that carried the agglutinogen 14 also absorbed antibodies against, B. parapertussis (Table 5). In conclusion, these results support the necessity to revise the subdivision of the genus Bordetella into three species. A change of B. parapertussis to B. pertussis within the epidemiological processes is considered.