The potential relationship between serum PRL levels and protein composition of breast secretions was evaluated in 54 premenopausal nonlactating women during the luteal phase of their menstrual cycle. Women were classified into four groups according to the presence or absence of breast pathology and to the protein pattern of their breast secretions. Type I mammary fluids contain Zn-alpha 2-glycoprotein, apolipoprotein D, and gross cystic disease fluid protein-15, whereas Type II fluids are characterized by the presence of some milk proteins such as lactoferrin, lysozyme, and alpha-lactalbumin. Basal serum levels of PRL, as well as of progesterone, LH, FSH, TSH, T3, and T4 were within normal range, and no significant differences were found between the different groups of women under study. However, after a TRH stimulation test, the maximum PRL response was significantly higher (P < 0.02) in normal women with Type II secretions than in those with Type I (64 +/- 6.8 micrograms/L vs. 43.7 +/- 3.9 micrograms/L). Similarly, when PRL concentrations in patients with benign breast disease were considered, those with breast fluids containing milk proteins had a rise in PRL secretion after TRH stimulation significantly higher (P < 0.05) than those with fluids lacking these proteins (77.1 +/- 6.2 vs. 58.8 +/- 5.1 micrograms/L). These results indicate that the occurrence of milk proteins in breast secretions from nonlactating women is associated with an increase in serum PRL concentrations after TRH stimulation, and opens the possibility of using breast fluid protein analysis as a simple and noninvasive procedure for studies on the putative role of PRL in the development of benign and malignant breast diseases.