In response to an estrogen, confluent monolayers of MCF-7 cell cultures develop multi-cellular nodules, termed foci. Post-confluent development of foci occurs with physiologic levels of 17beta-estradiol and are inhibited by various anti-estrogens acting through either the estrogen or aryl hydrocarbon receptors. In the present paper we report that disruption of the terminal sugars on membrane receptors results in inhibition of foci. Treatment with 0.013-0.05 units/ml of beta-galactosidase completely inhibited the development of foci while leaving the monolayer of cells intact. Trials with alpha-mannosidase resulted in a similar but less potent inhibition of foci. Lectin-fluorescent conjugates, RCA (Ricinus communis agglutinin), and ConA (Canavalia ensiformis agglutinin) were used to identify membrane surface carbohydrates on MCF-7 cells. Binding of the RCA-fluorescent conjugate was inhibited by co-treatment with galactose or lactose. Binding of ConA-fluorescent conjugate was significantly inhibited by mannose and n-acetyl-glucosamine. This is the first report of inhibition of foci development in MCF-7 cell cultures by disruption of surface carbohydrates on membrane receptors.