The stroma of ductal infiltrating carcinoma of the human breast shows characteristic and localized areas of collagen rarefaction and fragmentation. This finding has been correlated with a peculiar type of fibrillar damage, observed in a small percentage of collagen fibrils isolated in the native state from the tumour stroma. The same pattern of lesion has been reproduced in vitro by human collagenase digestion on reconstituted fibrils. No effect has been detected by other nonspecific proteases in the same system.