Leukemoid-reaction inducing factor was searched in mice with myelogenous type of leukemoid reaction induced by MY sarcom. The following results were obtained. 1) Leukemoid reaction was not induced by subcutaneous injection of the extract of MY sarcom. Tissue culture of bone marrow of mice injected with the extract did not show difference from the controls. Therefore, it was presumable that the extract of MY sarcom did not contain leukemoid-reaction inducing factor. 2) Serum of mice with leukemoid reaction induced by MY sarcom was demonstrated to produce hyperplasia of the bone marrow of normal mice. This seemed to indicate that serum of the mice with leukemoid reaction contained leukemoid-reaction inducing factor.