The BC-1 strain DNA of Marek's disease virus type 1 (MDV1) at high-passage in culture was found to contain tandem direct repeats of the complete long terminal repeat (LTR) sequence of avian leukosis virus RAV0 strain as the repeat unit within the short inverted repeats of the MDV1 DNA. Since the attenuated BC-1 strain grows well in cultured cells, the insertion site for retroviral DNA sequence within the short inverted repeat of MDV1 DNA is not essential for viral growth in culture.