Single and multi-jet liquid impingers and membrane-filters were found to be efficient sampling devices for aerosols generated from suspensions of the virus of foot-and-mouth disease (FMD). Concentration of the aerosol samples with an adsorbent, Attaclay, facilitated the detection of small amounts of virus. Sodium dodecyl sulphate could be used for elution as the virus of FMD is resistant to this anionic detergent.The penetration of these aerosols through various air filtration media was determined using impinger samplers. A glass fibre paper was found to be the most efficient with a virus penetration of less than 0·001%.We wish to thank Dr D. W. Henderson and Dr H. M. Darlow, of M.R.E. and Mr R. G. Dorman of C.D.E. Porton for valuable prefatory discussions and advice. We are also indebted to Miss S. Cartwright for inoculating mice, to Miss M. Shoobridge and Mr S. Taylor for technical assistance, and to Vokes Ltd. for supplying materials.