Primary cultures of bovine fibroblast (BF) and canine brain cells, persistently infected with virulent R252-canine distemper virus (CDV), were cocultured with African green monkey (Vero) cells. Transfer of persistent CDV from BF to Vero cells varied inversely with the in vitro passage level (age) of the CDV-infected BF cells. Successful transfer of CDV to Vero cells was signaled by the transient appearance of viral syncytia, rapid spread of viral antigen to all Vero cells in the culture, and by recovery of cell-free Vero-infectious virus in culture fluids. With time, viral cytopathic effects in Vero cells containing CDV disappeared, and the infected lines could not be distinguished from noninfected control Vero cells, except by immunoassay for viral antigen.