Tumor growth is the result of combined cell proliferation overwhelming cell death and neoangiogenesis. This report shows CAM angiogenesis promoted by TA3 tumor supernatant with or without low dosis of betamethasone (Minimal antiangiogenic concentration: ß-MAAC). Methylcellulose discs instilled with 10 µl of ß-MAAC (0.08 µg/ml),10 µl of tumor supernatant(TA3ts),5 µl ß-MAAC + 5 µl TA3ts, and 10 µl of PBS as control were implanted in host chick eggs. On day 12, the grafts were removed, photographed and fixed. Sections were stained in parallel, one and three with hematoxylin-eosin, and section two by the Tunel method. The number of vessels was evaluated in a microscopic field of the CAM (2250 µm2 ). The results show that ß-MAAC produced a significant inhibition of neovascularization in comparison to that observed in controls (P < 0.0025; Student t-Test). Discs instilled with TA3ts produced an intense stimulation of angiogenesis in contrast, when discs were instilled with 5 µl of ß-MAAC + 5 µl of TA3ts the angiogenesis was significantly inhibited (P< 0.001). The results show that effective antiangiogenic doses of betamethasone are in the range of 10-7 M, (probably a genomic mediated action) and that this effect of low concentration may have clinical applications.