Bovine retinas contain a factor that stimulates proliferation of aortic endothelial cells in culture as well as neovascularization on the chicken chorioallantoic membrane. The stimulatory activity has been partially purified from a balanced salt solution extract of bovine retinas. The stability of the activity to acid pH was utilized as the first step in purification. The acidtreated material was subjected to ion-exchange chromatography on DEAE Bio-Gel A. The material thatwas eluted with 0.