A simple reverse transcriptase-polymerase chain reaction (RT-PCR) assay has been developed for the specific amplification of DNA after reverse transcription of RNA from the classical swine fever virus (CSFV). A pair of oligonucleotides was selected from an area of high homology in the genome of CSFV strains, but which differed from the corresponding sequences in the genome of bovine viral diarrhea virus (BVDV) strains. Using these primers (CSFV1-CSFV2), a CSFV specific DNA band of 174 bp was amplified from the CSFV RNA extracted from four reference strains and 14 field isolates, as well as from 25 organ extracts and eight buffy coats and serum samples of experimentally infected animals. No amplification was observed with the RNA from four BVDV reference and vaccine strains and seven field isolates. This RT-PCR assay made it possible, in a one-step reaction, to detect CSFV rapidly, sensitively and specifically in cell culture supernatants and in clinical specimens.