The pH of 106 human breast cyst fluids has been measured immediately following aspiration. Values ranged from pH 6.3 to 7.8. They correlated with the ratio of Na+ to K+ in cyst fluid and differed significantly in the two populations of fluids which can be defined on electrolyte composition. Storage of cyst fluid for one month at -20 degrees C resulted in an increase in pH by a mean of 0.9 of a unit. pH provides a simple method for typing cyst fluids and this may be of clinical importance as there is evidence that there are differences in the natural history and association with breast cancer in the two populations of breast cysts.