Retinae from neonatal mice exposed to prolonged hyperoxia (100% oxygen), show marked vasoproliferation. Extracts of such retinae were chromatographed on a heparin-Sepharose column and the absorbed material subjected to HPLC fractionation. Two components, approx. 10 and 18 kDa, respectively, were found to have angiogenic activity, which was higher than in corresponding extracts from animals exposed to air. Both fractions and an additional 5 kDa component reacted with an antibody to basic fibroblast growth factor (bFGF) and showed higher levels in hyperoxia. The data suggest that hyperoxia activates angiogenic factors belonging to the heparin binding family.