IBR virus was found to replicate in WI-38 cells. At a high input multiplicity the virus yield was comparable to that obtained in bovine cells, but comparable degree of CPE took longer to achieve. At a low input multiplicity of IBR virus, such as may be encountered in virus contaminated bovine serum, virus yield was only about 1% of that in bovine cells, with 50% of the cells showing CPE, followed by cell regrowth. Infectious virus was not recoverable from the regrown cells by 5 weeks after initial infection, and these regrown cells were susceptible to reinfection with IBR virus. Aging of WI-38 cells in cultures for as little as 1 week reduced IBR virus yield to 90% less than the yield from the same lot of cells inoculated 7 days earlier.