Abstract. Cell lines initiated from a fibrocystic hyperplasia and a carcinoma of the breast and an established human fetal lung fibroblast line. MRC5, were grown in culture. The surface proteins of the cells were iodinated in cell monolayer, and in suspension following harvesting from culture flasks using EDTA. Data are presented which show that cells labelled in suspen sion show reduced incorporation of l25I into surface proteins of high molecular weight (> 100,000 daltons) and considerably increased incorporation into proteins with molecular weight < 100,000 daltons. These changes are discussed in relation to the possible release of surface material by the action of EDTA. It is also suggested, on the basis of the current findings, that the detachment of cells may expose low molecular weight components possibly involved in the attachment of the cells to the substratum.