AbstractFibrin deposition and removal is a feature common to major pathological processes such as wound healing, chronic inflammation and tumour invasion: processes involving the ingrowth of new blood vessels. Low molecular weight fibrin degradation products (MW<50 000) are now shown to induce angiogenesis in the chick chorioallantoic membrane (CAM). This effect has also been shown by new quantitative assays to be associated with stimulation of both DNA and protein synthesis. Autoradiography indicates that all cell types in the CAM are stimulated to divide, and it is proposed that fibrin degradation products are a pathological growth factor.