Experimental studies were performed to see whether Japanese quail (Coturnix coturnix japonica) are susceptible to JM strain of Marek's disease virus (MDV). In three identical trials, a total of 120, one-day-old quail were inoculated intra-abdominally with 0.2 ml. of chicken blood infected with MDV (JM strain) and raised in FAPP isolators. Uninfected controls were inoculated with normal saline only and raised separately. During the 18-week post infection observation period, 65-80% of the infected quail died within six weeks, while the highest mortality in control groups was only 0-5%. In each trial, ocular lesions with or without unilateral or bilateral blindness and signs of torticollis were evident in a few quail after 10-14 weeks. Gross and microscopic lesions suggestive of MD were observed 14 days on. The most pronounced lesions were observed in lungs. Neural tissues were least affected. In general, females were more susceptible than males. MD-specific fluorescent and precipitating antigens were detected in different visceral tissues, buffy coat and cultured macrophages of infected quail. Fluorescent antigen appeared at 6-7 days after infection, whereas precipitating antigen appeared after 12-15 days. Viral (MDV) infectivity tests in cell cultures and bioassay in one-day-old chicks and quail demonstrated that infected quail become viremic around the seventh day post infection. A cytopathic agent similar to MDV was also isolated from quail. Neither Newcastle disease virus nor a bacterial agent was isolated from the quail. MD-specific fluorescent precipitating antibodies were present in the egg yolk and plasma of both infected sick and infected symptomless quail. The earliest detectable plasma MD antibody appeared in 14-21 days. Such findings were not observed in quail from parent stock and controls. Our studies demonstrated that Japanese quail are susceptible to JM strain of MDV.