We investigated the presence of colony-stimulating factor (CSF) in supernatants obtained from TS/A, a new metastatic murine cell line, and from its high-and low-metastatic clonal derivatives (E and F clones, respectively). TS/A cells produced a CSF in vitro that induced proliferation and differentiation of murine monocytic and granulocytic progenitors in agar cultures. In TS/A-bearing mice remarkable splenomegaly, blood granulocytosis and thymus depletion were observed along with a stimulatory activity in serum and a strong proliferative activity both in spleen and in bone marrow populations. Conditioned media from E clones showed an in vitro colony-stimulating activity greater than those of F clones. Mice injected subcutaneously with cells of all clones studied showed granulocytosis, splenomegaly and thymus depletion, although to varying degrees. However, no direct correlation between granulocytosis-splenomegaly and the number of spontaneous lung metastases was observed.