Cyst fluids aspirated from patients presenting with palpable cysts of the breast were analysed for mutagenic activity, as determined by the Ames test. Of the 439 women studied, 9.6% had mutagen-positive fluids. There was no obvious relationship between the presence of mutagen-positive fluids and clinical or epidemiological details of the women from which they were obtained. Preliminary characterisation of the mutagenic activity suggested that the responsible agents have diverse chemical properties.