Murine myeloid leukemia secretes a substance immunochemically cross- reactive with insulin (SICRI) both in vivo and in serum-free media. High SICRI concentrations in peripheral blood of tumorous animals do not affect circulating glucose levels. In culture, DNA synthesis rate per leukemic cell is proportional to cell density and is reduced by antiinsulin serum. Culture medium conditioned by leukemia cells as well as SICRI affinity purified from this medium stimulate DNA synthesis in cultured leukemia cells. It appears that autocrine stimulation of murine myeloid leukemia can be mediated in part by an insulin-related growth factor.