Different cell tissue cultures and commercial fetal calf sera (FTS) used in biological and virological research were screened for the bovine viral diarrhea virus (BVDV, Pestivirus genus, Flaviviridae family) and mycoplasma contamination. BVDV was detected using RT-PCR and Indirect immunofluorescence (with monoclonal antibodies) methods in 33% cases of the studied cell lines and in > 60% cases of FCS. BVDV was shown to present and reproduce in high spectra of human cell lines, as well as in monkey, pig, rabbit, goat, dog, and cat cells at high levels (up to 100-1000 genome-equivalent copies per cell) and reached up to 10(3)-10(7) genome-equivalent copies per serum ml. The molecular mechanisms of the long virus persistence without definite signs of destruction should be studied.