Serological and cultural investigation of broilers with respiratory disease implicated Mycoplasma synoviae infection in the syndrome which was characterised by variable mortality up to 20% at 6-8 weeks of age in association with Escherichia coli septicaemia. Preliminary experimental studies using specific-pathogen-free birds confirmed that the Mycoplasma synoviae concerned was of the respiratory type, first described in the USA as causing a rapidly spreading, persistent upper respiratory tract infection associated with production losses and in some flocks condemnations due to air sacculitis.