LETS is a large surface glycoprotein that is found on normal fibroblasts, but is absent or exists in amounts on transformed cells. Immunofluorescent staining shows LETS protein fibrils arrayed around the cells, particularly concentrated beneath the cells and in the area between neighboring cells. LETS glycoprotein is disulfide-bonded at the cell surface into dimers and higher aggregates. Other surface proteins also appear to participate in disulfide bonding. Reduction of disulfide bonds leads to increased release of LETS protein from the cells, as does the addition of cytochalasin B. Purified LETS protein added to transformed cells binds to the cells in a fibrillar array similar to that seen on normal cells. Addition of LETS protein leads to increased attachment and spreading of cells and causes transformed cells to align like normal ones. It also causes the appearance of actin cables in transformed cells, which normally lack them. These effects are inhibited by specific antisera to LETS protein or by reduction of disulfide bonds in the protein and are blocked or reversed by proteolysis. The results suggest that LETS protein plays a role in adhesion of cells.