The coloration of nipple aspirates of breast fluid from 2,343 women were compared with breast fluid concentrations of cholesterol. cholesterol epoxides, lipid peroxides, estrogens, and immunoglobulins. Darker-colored breast fluid had significantly higher mean concentrations of cholesterol, cholesterol epoxides, lipid peroxides, and estrogens than did lighter-colored fluids, whereas no relationship of coloration with immunoglobulin levels was found.