The African green monkey kidney fibroblast-like cell line has been established from CV-1 cells which have been transformed by an origin-defective mutant of SV40 coding for wild-type T antigen. This line contains T antigen, retains complete permissiveness for lytic growth of SV40, supports the replication of ts A209 virus at 40°C, and supports the replication of pure populations of SV40 mutants with deletions in the early region.