Measurements of the spectral distribution of x-rays using a Si(Li)-detector form the basis for an investigation into contrast values provided by molybdenum and tungsten tubes in mammography. In order to evaluate contrast, the intensity of the characteristic radiation and of the brems rays were related to anode kV and thickness of section; the spectral intensity was then calculated for small calcium inclusions in soft tissues. For average tissue thickness (4 cm.) and an anode working at 30 kV, the molybdenum tube with a molybdenum filter is significantly better than a tungsten tube. With increasing size of breast, hardening of the radiation also increases, until for practical purposes contrast is determined by the brems rays alone. At this stage there is only little difference between the molybdenum and tungsten spectrum, and in the resultant contrast.