Classification, by agglutinogens, of 634 isolates of Bordetella pertussis collected from 1941 to 1968 in Great Britain demonstrated that a change from a predominantly 1,2,0,4 serotype (75% of those examined during 1941–4) to a predominantly 1,0,3,0 serotype (73% of those examined during 1966–8) occurred sometime after 1953. Furthermore, evidence from the examination of isolates collected between 1941 and 1953 suggests that the change may have been gradual. Isolates of serotype 1,2,3,4 made up 20–30% of the total of our cross-country selection for the periods 1941–4, 1946–9, 1950–3 and 1966–8, but over shorter periods in individual areas the percentage varied from negligible to as high as half of those isolated. Results from other countries show a similar drift towards a 1,0,3 serotype but more often from a 1,2,3 than from 1,2,0 serotype.
The value, in epidemiological studies, of extended information obtained by monospecific typing sera to all six, rather than only two or three agglutinogens, and confirmation of the results by agglutinin production is demonstrated: for instance not all 1,0,3 isolates were identical.