PURPOSE: To obtain an antitumor active substance in a large amount with hardly any side effects due to the substance derived from living bodies without limitation on the amount of a fed raw material, by cultivating a mouse growth cartilage-derived cell strain, collecting a substance having inhibitory activity against vascular endothelial cell growth from the resultant culture.

CONSTITUTION: Mouse growth cartilage tissues are treated in 0.1% EDTA, washed and successively treated with 0.15% trypsin and then 0.15% collagenase to separate a mouse growth cartilage-derived cell strain, which is subsequently cultivated in a culture medium containing 20% bovine fetal blood serum. The culture fluid is recovered to remove cellular components by centrifugation and provide a cultivation supernatant liquid, which is then dialyzed and freeze-dried to afford the aimed antitumor active substance having inhibitory activity against vascular endothelial cell growth.