Epidermal growth factor (EGF), dehydroepiandrosterone (DHA)-sulphate and [Na+] and [K+] were assayed in 78 cyst fluids from patients with a palpable breast cyst. Epidermal growth factor was detected in all but 2 cysts, the mean value +/- SEM being 506.2 +/- 39.3 ng/ml, with a range of 0-1,599 ng/ml. When the cyst fluids were sub-divided according to their [Na+]:[K+] ratio, group A cyst fluids ( [Na+]:[K+] less than 3) had a significantly higher (p less than 0.001) level of EGF than group B cyst fluids ([Na+]:[K+] greater than 3). Furthermore, the relationship between EGF and [Na+] and [K+] and between EGF and DHA-sulphate seemed to differ between the 2 cyst types and each cyst type was therefore analyzed separately.