The JLS-V9 bone marrow cell line was infected with Moloney leukemia virus (MLV). A new surface antigen was detected on the infected cells by indirect membrane immunofluorescence and immune adherence to living target cells. The serum of MLV-immunized mice was used as the reagent. The time course of antigen expression was studied with various sources and dilutions of the virus. A virus preparation was also compared in tests for leukemogenic and immunogenic activity in vivo and antigen-inducing ability in vitro. The tissue culture test provided a useful tool to assess MLV infectivity.