To study the sites of action of cytokines and anti-angiogenic agents, bioassays have been developed to systematically investigate not only the entire process of angiogenesis in vivo but also its individual steps including capillary tube formation and proliferation of vascular endothelial cells (EC). Angiogenesis in vivo is quantitatively assayed by measuring the carmine dye content in Freund's complete adjuvant-induced mouse pouch granuloma after the intravenous administration of dye solution. Angiogenesis in vitro is quantified by counting the number of microvessels developed from blood vessels cultured in fibrin gel. Tube formation is quantitatively estimated by measuring the total length of capillary tubes from EC cultured in type I collagen gel. EC proliferation is assayed by measuring both the increase in cell number and 3H-thymidine incorporation into the cells. The competence and progression activities in the EC proliferation are analyzed by our convenient method. By these bioassays, the mechanism of an anti-differentiation agent can be easily clarified to develop new types of therapies for rheumatoid arthritis and diabetic retinopathy.