The effects of Friend erythroleukemia cells on angiogenesis were studied in chick embryo chorioallantoic membrane assay and in human umbilical vein endothelial cells. In chorioallantoic membrane assay, the conditioned medium of Friend cells stimulated in vivo angiogenesis to an extent comparable to that observed with Prostaglandin El, used as positive control. Prostaglandin El added to conditioned medium of Friend cells did not further increase angiogenesis. Conditioned medium of Friend erythroleukemia cells also stimulated proliferation of human umbilical vein endothelial cells to an extent comparable to that observed with fetal bovine serum, used as positive control. Conditioned medium and fetal bovine serum together did not affect human umbilical vein endothelial cells proliferation, as compared to that observed when tested separately. These results seem to indicate that Friend erythroleukemia cells produce and secrete factors stimulating angiogenesis. These findings extend and confirm the hypothesis that successful angiogenesis is necessary for development of leukemias.