The invention relates to a multiplex real-time RT-PCR assay with a heterologous internal control system (i.e., EGFP-RNA) for the simple and fast diagnosis of classical swine fever virus (CSFV). Primers and FAM-labeled TaqMan probes, specific for CSFV were selected by analyzing the consensus sequence of the 5'-non translated region of various CSFV strains. For determining the analytical sensitivity an in vitro transcript (T7-PC3Alf) of the 5' NTR was constructed and tested. Furthermore, a primer-probe system for the detection of the internal control sequence was established, and a multiplex assay using CSF-System 1 and the IC real-time PCR could be performed as a one-tube assay without loss of sensitivity or specificity.