Endothelin-like immunoreactivity in human breast cyst fluid. Immunoreactive endothelin has been detected in 21 of 43 samples of breast cyst fluid (21 cases; 3.5 +/- 0.6 pmol/l, mean +/- SEM. Other 22 cases; not detectable, less than 0.5 pmol/l). Fast protein liquid chromatographic analysis of the immunoreactive endothelin of pooled breast cyst fluid showed two immunoreactive peaks; one in the void volume and the other in the position of endothelin-1. It is probable that endothelin-1 is produced by the epithelial cells lining breast cysts, but significance of the presence of endothelin-1 in breast cyst fluid remains to be elucidated. 