Mouse myeloma cell line (SLU-5) that has undergone lengthy propagation in vitro and become nononcogenic was compared with an earlier oncogenic passage of cells. Immunogenicity of cells cultured for various periods of time was compared with that of the original tumor (MOPC-21). The nononcogenic cells were most immunogenic. Cells that were nononcogenic in normal mice produced tumors in nonlethally irradiated mice. Clones isolated from oncogenic and nononcogenic populations varied widely with respect to ability to produce tumors in mice and to specific globulin production.