While infecting VMD-Kekava chick fibroblasts cultures two plaque-forming variants of the virus were isolated. Both variants are cell-associated viruses; during a prolonged period these are circulating in the blood of infected chicks, including also healthy ones. The cultures infected contain a common precipitating antigen; sera from the infected chicks have antibodies to VMD feather antigen, both viruses produced, although in a different degree, in the infected chicks lymphoid infiltration of nerve trunks typical for Marek's disease. These variants differ one from the other in the type of plaques formed by them in the tissue culture, and in their pathogenicity for chicks. Variant 83 is likely to be naturally attenuated virus, and further on it may be investigated as a vaccine strain.