In a part of the broiler flocks vaccinated against Newcastle disease (N.C.D) and infectious bronchitis (I.B.), disease symptoms of lingering nature have been observed, generally in the second half of the rearing period. In a practical investigation with weekly examinations of chickens, supplemented by a serological examination of twenty-four animals per flock at the age of six weeks, it was hoped to establish the factors responsible for this "vaccination reaction". In the district under notice the vaccination reaction syndrome had been responsible for widspread abandoning of twice spraying against N.C.D. in the first and fourth week in favour of drinking-water vaccination (generally combined with I.B. vaccination) in the second week and spray-vaccination in the fourth week or of combined drinking-water vaccination in the second week, either with or without N.C.D. drinking-water vaccination in the fourth week. Admittedly the incidence of vaccination reaction in flocks vaccinated exclusively via the drinking water was less frequent (32%) than in flocks in which the second vaccination was administered as a spray (48%), but this difference was largely accounted for by infection with Mycoplasma gallisepticum and/or Mycoplasma synoviae, complicated by infection with Escherichia coli. Of the thirty-three flocks free of mycoplasmosis, 24% exhibited the vaccination reaction, while the incidence to the fifteen infected flocks was 73.4%. In flocks infected with mycoplasmosis the course of E. coli infections was serious in 46.7% of the birds, while this figure was 18.2% in flocks free of mycoplasmosis. In flocks free of mycoplasmosis, the percentage of serious E. coli infections was lower after spray vaccination (14.3%) than after drinking-water vaccination (21%). The strains of E. coli brought in by one-day chicks from the hatchery disappear rapidly and play no role of any significance in the problem of colibacillosis observed at an age of three weeks or older. Of the 310 isolated strains of E. coli, 52 could not be typed and the others belonged to eighty different serotypes. With respect to the effect of infectious bronchitis the investigation does not provide sufficient evidence to permit of drawing conclusions. A significant role in the occurrence of the syndrome was played by coccidiosis and Gumboro's disease. With respect to environmental factors the available data did not allow of drawing conclusions. The authors recommend continuing with all available means to free the breeding animals from M. gallisepticum and M. synoviae, undertaking scientifically based research into the role of infectious bronchitis in the "vaccination reaction" syndrome, an effective programme of hygiene to control E...