Computer linked morphometric methods and the measurements of DNA synthesis were used to quantitate angiogenesis on the chorioallantoic membrane (CAM) of chick embryos. The former measured changes in vascular parameters such as the length and number of arterial branches whereas the latter measured the degree of DNA synthesis associated with angiogenic stimulation of the CAM. Time-course graphs prepared using the DNA synthesis assay showed that fibrin degradation products (FDPs), histamine and heparin took 18, 36 and 12h respectively, to stimulate maximal DNA synthesis. Therefore it was suggested that these substances employ different modes of action to stimulate angiogenesis on the CAM. It appears that histamine is an indirect stimulator of angiogenesis but the heparin results are from preliminary experiments and further work will be needed to verify them. Tumour angiogenic factor produced a 200% increase in DNA synthesis, 18h after its application so it appears to act like FDPs. Fibronectin degradation products were found to be involved in the FDP response and since fibrinogen, its degradation products, bovine serum albumin and fetal calf serum were not active it appears that the angiogenesis induced by the FDPs was specific to a product of cross-linked fibrin.