Normal and neoplastic breast tissues have been characterised in terms of x-ray attenuation. Samples of normal fat and fibrous tissue were obtained from reduction mammoplasty and autopsy, and infiltrating duct carcinoma specimens from mastectomy and lumpectomy. A high-purity germanium spectroscopy system and a beam of 120 kV constant potential x-rays were used to determine the linear attenuation coefficient from 18 to 110 keV. Densities were determined from buoyancy measurements and were used to obtain mass attenuation coefficients. Infiltrating duct carcinomas and fat are well distinguished by x-ray attenuation. For photon energies used for film-screen mammography, infiltrating duct carcinomas are more attenuating than fibrous tissue. Above 31 keV, the ranges of attenuation of the two tissue types overlap. The attenuation coefficients of tissues have been concisely represented by equivalent thicknesses of lucite and aluminium. Analysis based on the average attenuation properties of tissues indicates that dual-energy mammography, using an ideal imaging system, would require 0.06 cGy to provide images in which 1 cm infiltrating duct carcinomas are displayed with a signal to noise ratio of 5 against a background over which the fat/fibrous contrast has been suppressed. This dose is similar to that currently used in conventional film-screen mammography.