SUMMARYThe WM1‐B strain of Friend virus was originally recovered from mice which had been inoculated with WM1 virus. The question of relationship between the two strains was examined.Antisera prepared in stock C3H mice against either WM1‐B or WM1 virus neutralized WM1‐B virus but not WM1 virus. Q.I.M.R. mice infected with WM1 virus were resistant to superinfection with WM1‐B virus. These two findings indicated that a relationship existed between some component of the WM1 line and the leukaemogenic component of the WM1‐B line.By treatment of WM1‐B virus with C3H antiserum to WM1 virus, a strain of WM1 virus was obtained which did not produce antibodies to WM1‐B virus in C3H mice and did not protect Q.I.M.R. mice against challenge with WM1‐B virus, but which did produce the lymphoid hyperplasia typical of WM1 virus infection. This pure WM1 strain was still free of relationship to WM1‐B virus at the 8th passage.The original WM1 line therefore contained two viruses. One (WM1 virus) caused lymphoid hyperplasia in Q.I.M.R. mice, while the other (an avirulent variant of WM1‐B virus) caused no signs but was detectable by means of neutralization and protection tests.Seventeen wild mice were studied virologically. Strains of virus resembling pure WM1 virus were isolated from two wild mice, and an avirulent strain related to WM1‐B virus was obtained from a third, while both viruses were isolated from three other wild mice.These results support the view that WM1 virus and the avirulent form of WM1‐B virus are distinct.It was concluded that an avirulent virus, which is apparently related to Friend virus, occurs in wild mice in Queensland.