The occurrence of a specific glucocorticoid binding activity was investigated in breast cyst fluid (BCF) samples aspirated from 481 women under treatment for breast cystic disease. [3H]cortisol was incubated with BCF at 4 degrees C with and without 100-fold molar excess of non-radioactive steroid in order to account for non-specific binding. Corticosterone, progesterone and dexamethasone-binding activities were also investigated in about 50% of the specimens. Consistent amounts of a specific cortisol-, corticosterone- and progesterone-binding component were observed in about 30% of samples. Apparent binding values ranged from 4.31 to 80.78 nmol/l for cortisol, from 3.94 to 75.72 nmol/l for corticosterone, and from 1.18 to 16.45 nmol/l for progesterone. No specific binding for dexamethasone was detected. Scatchard analyses for [3H]cortisol and [3H]progesterone were made in pools of 'positive' BCF, human serum and whey of human milk, respectively. The mean values of the apparent equilibrium association constant (Ka) were compatible with the existence of high affinity protein-hormone interactions in each considered medium. Data obtained in ligand competition experiments for different steroid molecules suggest the similarity of the glucocorticoid- and progesterone-binding component found in 30% of BCF samples examined with the plasma corticosteroid-binding globulin (CBG, transcortin) and especially with the transcortin-like component detected in the whey of the human milk. It is suggested that the transcortin-like component may play an important role in controlling kinetics of transport and in regulating the effective levels of cortisol and progesterone in numerous breast cyst fluids.