Putative cell-surface proteins of tissue-culture cells were identified by lactoperoxidase-catalyzed iodination, a technique that attaches label only to proteins outside the cell membrane. Evidence is presented that these proteins are cell derived, not contaminating serum proteins. On "normal" cells, which exhibit density dependence of growth, one protein of high molecular weight was particularly readily iodinated. This protein was easily removed by mild proteolytic digestion, and, in virus-transformed cells, was either absent or unavailable for iodination. The possible relevance of these observations to the control of growth in cell culture is discussed.