During epidemics of classical swine fever (CSF), the route of virus introduction into a farm is often unclear. One of the suggested routes is via the air. Under experimental conditions, airborne transmission over a short distance seems possible, but analysis of outbreak data is still inconclusive. For a better understanding of the role of airborne transmission, quantitative information is needed on concentrations of virus emitted by infected pigs. This was studied in four groups of 10 pigs in which three pigs were inoculated with either a low virulent strain (Zoelen), a low or high dose of a moderately virulent strain (Paderborn), or a highly virulent strain (Brescia). The other seven pigs in each group served as contact pigs. At several moments after infection, air samples were obtained using gelatine filters. Infectious virus and viral RNA were detected in the air of rooms housing the pigs infected with the moderately and highly virulent strains with titres of 10(1.2) to 10(3.0)TCID(50)/m(3) of infectious virus, and 10(1.6) to 10(3.8)TCID(50)equiv./m(3) of viral RNA. It was observed that the higher the dose or virulence of the virus strain used for inoculation of the pigs, the sooner virus could be detected in the air samples. This is the first study describing the quantification of (infectious) CSFV in air samples of rooms housing infected pigs, enabling to quantify the contribution of individual infected pigs to virus concentrations in aerosols. This can be used as input for quantitative models of airborne spread over large distances.