75 patients with breast gross cystic disease and no cancer have been included in the study. For each patients serous and intracystic concentrations of MCA have been measured. The aim of the study is to assess whether if a relation between intracystic concentration of the marker and resistance and capability of cellular reproduction exists (confirmed by the release of the cyst). The analysis of intracystic values shows that synthesis of MCA is an intrinsic peculiarity of cytologic kind. It is apparently independent from inflammatory or hemorrhagic contemporary processes.