Untreated EMT6 tumors and bone marrow of tumor-bearing BALB/c mice secrete factors capable of stimulating quiescent medullary splenic colony-forming units into cycle. Since the tumor population is heterogeneous, it is of interest to determine the nature of the cells which are involved in this phenomenon. Experiments with conditioned media from tumors of BALB/c and athymic mice show that EMT6 cells themselves may secrete a stimulating factor but that the presence of T-lymphocytes is necessary at some time in the development of the tumor. Likewise, the presence of the tumor is necessary for bone marrow to secrete these factors in our experimental model. This suggests reciprocal influences of the T-cells and the EMT6 tumor cells in the phenomena that we describe.