Bovine leucocyte extract containing the granulocytic chalone inhibited proliferation of rapidly, but not slowly, prliferating murine CFU-C. The precursors of CFU-C, i.e. multipotent colon-forming stem cells (CFU-S), were apparently not affected. Inhibition of proliferation was detected as a decreased killing of DNA synthesizing cells in diffusion chamber cultures by hydroxyurea, after pre-treatment with the extract.