Serum proteins and lipoproteins were determined in 23 menopausal females after surgery for early forms of breast cancer and the results compared with data from a matched group of randomly selected healthy females. The patients were randomly divided into 2 groups, one serving as a control group, the other receiving 40 mg tamoxifen daily for 2 months. Breast cancer patients were found to have significantly higher concentrations of serum cholesterol than controls (7.90 +/- 1.15 vs. 6.87 +/- 1.18 mmol/l, P less than 0.001), which was the result of a 16% higher concentration in LDL cholesterol (P less than 0.05) and a 13% higher concentration in HDL cholesterol (P less than 0.05). During tamoxifen therapy total TG tended to increase, whereas total cholesterol fell. Significant lipoprotein changes were found in the LDL fraction where LDL-TG increased from 0.46 to 0.56 (P less than 0.01) and LDL cholesterol fell from 5.11 to 4.10 mmol/l (P less than 0.001). During tamoxifen therapy haptoglobin and orosomucoid concentrations fell significantly (P less than 0.01), whereas those of alpha-antitrypsin and ceruloplasmin increased (P less than 0.001). Factors such as diet and weight may explain the differences between breast cancer patients and controls. The tamoxifen-induced changes indicate that this anti-oestrogen exerted a mild oestrogen-like effect with regard to protein and lipoprotein metabolism.