In vivo suppression of neovascularization by in situ administration of human recombinant interferon alpha 2a (hrIFN-alpha 2a) was tested on the chick embryo area vasculosa (AV). Methylcellulose discs, each containing 5 IU of hrIFN-alpha 2a were implanted onto the AV at Hamburger-Hamilton (HH) stage 13, and inhibition of blood vessel growth was morphometrically evaluated between HH stages 20 and 27. Our results show: (i) a decreased extent of the AV; (ii) a decreased AV total vessel length; (iii) modified percent ratios of different classes of AV vessels having a definite length. The observed effects of treating with hrIFN-alpha 2a are likely attributable to inhibition of endothelial cell proliferation.