Two new classical swine fever virus (CSFV) isolates obtained from naturally infected swine were found to exhibit a cytopathogenic (cp) phenotype. According to their reactivity with monoclonal antibodies (mabs) the isolates cpBW1 and cpMVP1 were classified as antigenic types "Lothringen'92" and "Flandern'90", respectively. In Northern blot analyses and PCR assays CSFV RNA of subgenomic length was detected in infected cells indicating the presence of defective interfering particles. Nucleotide sequencing of cDNA fragments obtained by reverse transcription (RT)-PCR allowed to localize in the genomes of the cpMVP1 and cpBW1 DIs deletions of 4,764 and 4,746, respectively. From the latter isolate a noncytopathogenic (noncp) biotype was obtained by cloning in cell culture. CSFV cpBW1 and a noncp isolate were used separately for infection of pigs. Viremia occurred much quicker after infection with cp virus and clinical signs were more prominent compared to animals infected with noncp virus.