Foot-and-mouth disease (FMD) virus may spread in a number of ways: by direct contact between infected and susceptible animals, by animal products such as meat and milk, by mechanical transfer on people, non-susceptible animals, birds, vehicles and fomites, and by the airborne route. The initial pattern of outbreaks at the beginning of the epidemic in the West Midlands of England in 1967 suggested that spread was airborne. The meteorological evidence for this and for past epidemics in Great Britain has been investigated by L. P. Smith, P. B. Wright and M. Hugh-Jones (personal communications, 1968-9) and by Hurst (1968). Henderson (1969) has also studied the spread of disease in the Worcestershire area and attributed much of it to wind carriage. Methods and results of aerosol sampling of virus and infected cattle were reported by Thorne & Burrows (1960) and by Hyslop (1965). In this paper the results are given of the measurement of virus in aerosols produced by cattle, sheep and pigs infected with FMD virus.