Comment: The MCF 10A cell line is a non-tumorigenic epithelial cell line. The cells are positive for epithelial sialomucins, cytokeratins and milk fat globule antigen. They exhibit three dimensional growth in collagen, and form domes in confluent cultures. Thus far, the cells have shown no signs of terminal differentiation or senescence. The line is responsive to insulin, glucocorticoids, cholera enterotoxin, and epidermal growth factor (EGF). By electron microscopy the cells display characteristics of luminal ductal cells but not of myoepithelial cells. They also express breast specific antigens as detected by positive reaction with MFA-Breast and MC-5 monoclonal antibodies. The calcium content of the medium exerts a strong effect on the morphology of the cells.