Elimination of the RNase activity of classical swine fever virus (CSFV) glycoprotein E(rns) was previously shown to result in virus attenuation. Specific reduction of B cell numbers in the peripheral blood, a typical symptom of CSFV infection in pigs, was not detected on infection with the RNase-negative mutant C-H346Delta [Meyers et al. (1999). J Virol 73, 10224-10235]. The present report shows that this feature is restricted to this specific virus mutant, and does not represent a general property of RNase-negative CSFV. The effects induced by infection with two other RNase-negative and wild-type (wt) CSFV strains on the composition of peripheral blood cells have been further analysed. For all viruses, not only general leukopenia but also a reduction of different subsets of leukocytes (T-lymphocytes, monocytes and granulocytes) was detected. Similar to the results with B-cells, no significant differences with regard to changes in cell number were determined for RNase-negative mutants and wt virus during the initial phase of infection. Later, the values returned to pre-infection levels for the mutants, but stayed at low levels in the wt virus-infected animals. A major difference was reflected in the virus load of the infected animals, which was dramatically higher for pigs infected with wt CSFV, so that reduction of the virus load represents a further marker for attenuation resulting from RNase destruction. Attenuation was also detectable for the RNase-negative mutant C-W300G, which showed rapid reversion to the wt sequence within the infected pig. The prevention of fatal disease after infection with C-W300G is apparently determined during the short time between infection and reversion, as the virus revertant reisolated from infected pigs was shown to be virulent when used for infection in a follow-up study. Reversion of C-W300G was also detected in tissue culture during passage on swine testis epithelioid cells and porcine transformed kidney (MAX) cells, whereas the mutation was stable when SK6 or 38A1D cells were tested.