The biological characteristics of L783V/3T3 cell line were investigated. It was demonstrated that L783V/3T3 cells had no morphological and growth changes and no colonies formed in soft agar medium. Lymphocytic leukemia could be induced by inoculation of cell-free extracts from L783V/3T3 cells into Kunming strain newborn mice. The results indicated that there was no oncogene in L783-MuLV genome, L783-MuLV belonged to slow oncogenic virus.