5641 micro-calcifications in 100 mammary duct carcinomas were magnified 20 times and then classified according to their shape. Four readily definable configurations were recognised: punctate, bean-shaped, linear and branching. In 95% of cases at least two of these configurations were identified next to each other within individual groups (polymorphy). Monomorphic punctate micro-calcifications occurred almost exclusively in the small papillary/cribriform carcinomas measuring a few millimetres. Larger examples of this type of carcinoma showed a polymorphic picture which was, however, dominated by punctate micro-calcifications. Comedo carcinomas showed linear and branching patterns. This phenomenon can be explained by the histological structure of these two ductal carcinomas.