Mouse mammary papillomas elicit new formation of vessels when transplanted onto the rabbit iris. This angiogenic capacity is a property of carcinomas but not of the resting mammary gland. In mouse papillary hyperplasias, however, this property appears much earlier than any morphological or clinical sign of carcinoma. A test for angiogenic capacity may reveal a step in the progression toward clinical malignancy and thus could be used to screen for neoplastic potential of hyperplastic epithelium in biopsy tissues.