Zinc alpha-2 glycoprotein (ZnGP) was measured in human breast microcysts, breast secretions, breast cyst fluid and serum. Detectable amounts of ZnGP were found in all fluids but the highest levels were found in microcysts. Apocrine macrocysts had a higher ZnGP level than flattened macrocysts. In both cysts and secretions levels of ZnGP correlated with those of dehydroepiandrosterone sulphate. Levels were significantly higher in cyst fluids from women who developed further cysts during follow-up compared with those in fluid from women who did not. Concentrations of ZnGP in serum from breast cancer patients were significantly higher than controls but not women with breast cysts. Women with node positive breast cancer had higher serum levels compared with those in node negative patients. Women with more advanced breast cancer had higher serum ZnGP levels than those with earlier disease. ZnGP is a serum and breast marker of apocrine activity and may prove to be a useful prognostic marker in breast cancer.