In order to study the possible correlation between carcinoembryonic antigen (CEA) and cellular proliferation, we assayed the concentrations of this substance in the fluid of 77 bening macrocysts of the breast classified according to their Na+/K+ ratio and compared them with those of transforming growth factor beta 2. CEA levels correlated positively and significantly with the cationic ratio, the concentrations of albumin, glucose, Cl- and pH and were higher (range: 2.5-81.5, median 12.8 vs range: 0.4-41.5, median 3.2 ng/ml (p: 0.00000) in type 2 (Na+/K+ > 3) than in type 1 (Na+/K+ < 3) cysts. There was no correlation between CEA and TGFb2, nor between the former and dehydroepiandrosterone sulphate levels. These results led us to suggest that the high CEA concentrations in type 2 cysts seem to be the consequence of loos of cellular differentiation and disruption of the cyst wall lining as well as the acquisition of embryonary properties by the latter as a consequence of a reduced hormonal microenvironment.