Marek's disease virus (MDV) antigens, as detected by immunofluorescence, were induced in a lymphoblastoid cell line, MSB-I, in the presence of IdUrd. When treated with 20 microng/ml of IdUrd there was no increase in the number of cells producing virus particles. If IdUrd was removed, an increase in virus production followed. Activation of the MDV genome appeared to require incorporation of IdUrd into cellular DNA and occurred during the first 12 h of culture. Expression of the activated genome required de novo protein synthesis and occurred during the next 12 h. The MDV genome in high producer MSB-I cells could be activated with low concentrations of IdUrd, whereas low producer MSB-I cells could not be activated with IdUrd to any great extent.