Fixation of embryonic chick cells (heart, neural retina, and limb bud) in the presence of lanthanum ions shows the presence of an electron-opaque layer, about 50 A thick, external to the cell membrane. This layer, designated LSM (for lanthanum-staining material), is not removable by trypsin, pronase, EDTA, DNase, α-amylase, neuraminidase, or N-acetyl-L-cysteine. However, phospholipase C, in concentrations as low as 0.001 mg/ml, succeeds in stripping the LSM from the cell surface. Heating the enzyme preparation does not inhibit this activity, but removal of divalent cations does; both of these results are consistent with the known properties of phospholipase C. The LSM is present at the cell surface in the control tissues and on cells dissociated from the tissues by proteolytic enzymes and EDTA. These results are interpreted to mean that the LSM is probably an integral part of the cell and not an extraneous coat. How this phenomenon bears on the problem of cellular adhesion is discussed, as is the possible chemical composition of the LSM.

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