Considerable evidence has accumulated that Ig is present on lymphocytes where it acts as antigen specific receptor (1). We have recently described a method for the isolation and characterization of this cell surface Ig and have demonstrated that it is predominantly 8S IgM on mouse splenic lymphocytes (2). The purpose of the present study was to determine the fate of cell surface Ig in splenic lymphocytes from nonimmunized mice. This information would be helpful in eventually achieving an understanding of the effect of antigen interaction on the receptor and the mechanism by which this effect is translated into a signal for stimulation of the lymphocyte. Our results indicate that cell surface Ig turns over rapidly and is released during in vitro incubation. Materials and Methods. Preparation of iodinated splenic lymphocytes. Splenic lymphocytes from adult BALB/c mice were prepared and iodinated as previously described (2).