A persistent lymphocytic choriomeningitis virus, noninfectious for mice, was revealed in the continuous human cell line, Detroit-6. The virus was detected in the cell monolayer by an indirect immunofluorescence test and in the cell homogenate by a complement-fixation test. Between 30 and 80% of the cells produced viral antigen in subsequent passages of the culture. Thymidine analogues (BrDU and IDU) stimulated the synthesis of antigen. DNA of the persistently infected cell line was transfected to the mouse lymphoid cell line, Lmo, which became persistently infected with the virus.