Repeated injections with tritiated thymidine at 4-hour intervals were used to label all proliferating cells (P cells) in a transplantable rat rhabdomyosarcoma to distinguish these cells from nonproliferating cells (Q cells). After the last injection, the tumors were excised and tumor-cell suspensions were prepared. From these suspensions, cells were cultured for different time intervals up to 120 hours. Labeled cells and clones in cultures of different ages were counted by autoradiography; this revealed that Q cells from this rhabdomyosarcoma have an approximately egual capability as P cells to produce clones of at least 16 cells within 4 days after plating. The labeling index of cells in 12-hour cultures was equal to that of cells in the tumors, which indicates that the cell-dispersion technique applied to prepare a cell suspension from the tumor did not preferentially select P or Q cells. Factors which can induce Q cells to begin proliferation are discussed.