The present study deals with the growth behavior of BVD (Singer strain) virus in primary BK cel1s with regard to the rate of virus increase in cell free and ce11 associated virus, as well as detection of viral protein as assayed by Dot_ELISA, FAT and IPT. The results revealed the following: * The optimum time to harvest the cal] free virus was at 72 hours PI with titer (10 4-6), while for cell associated virus part its peak (l0 4) was 48 hours PI. * The maximal development of viral antigens could be traced and titrated by Dot ELISA. The most extensive reactions had been given by the ce11 free viral antigen 72 hours, its titer was found to be higher (26) than the intracellular (25) virus harvest 48 hours PI. * The FAT proved to be sensitive and reliable technique for detecting and tracing the development of viral antigens. The best time to detect clear intracytoplasmic fluorescence in BK cells was at 48 hours PI. * Regarding the comparative sequential studies between IPT and FAT it was found that both tests ran parallel concerning their sensitivity for detection and tracing of BVD viral antigens.