The cellular location of the src gene product (p60src) of the Schmidt-Ruppin strain of avian sarcoma virus has been determined by electron microscopic immunocytochemistry in Schmidt-Ruppin ASV-transformed NRK cells, and the amount of the protein in different regions of the cell has been quantified. The protein is concentrated on the inner surface of the plasma membrane, particularly under ruffles, and it is highly concentrated on the inner surface of the membrane near junctions connecting adjacent cells. Small amounts of p60src were detected in the cytoplasm and in the perinuclear Golgi region of the cell. No significant localization was detected in control NRK cells or in NRK cells transformed by the Kirsten strain of murine sarcoma virus. The presence of p60src on the inner surface of the plasma membrane indicates that the changes in cell growth, cell shape and cell membrane structure noted in ASV-transformed cells are due to an initial action of p60src at the cell membrane.