One hundred human breast cyst fluids have been analysed for sodium (Na+), potassium (K+) and dehydroepiandrosterone (DHA) sulphate. Concentrations varied greatly between individual cyst fluids, Na+ from 20 to 185 mmol/l, K+ from 5 to 160 mmol/l and DHA-sulphate from 1.5 to 87 mumol/l. Analysis of the inter-relationships between Na+, K+ and DHA sulphate revealed two major sub-populations of cyst fluids--one group in which Na+ levels were markedly in excess of K+ and DHA sulphate concentrations were low and the other in which K+ was the predominant cation and DHA sulphate concentrations were high.