We examined the specificity of the Ig “coating” on murine tumor cells grown in vivo. Cells were treated in vitro for release of cell-bound Ig from a.scites tumors. Such uncoated cells showed an increased expression of tumor-associated antigens and a parallel decrease in intensity of the Ig coat, but displayed no changes in the expressions of other normal membrane antigens. This was shown by a complement-dependent cytotoxicity assay and radioimmunoassay. These changes were attenuated if the tumor originated from animals that had been irradiated before tumor inoculation. No alterations were found with corresponding cells propagated in vitro and submitted to the same treatments. Our findings and others suggest tumor-specific antibodies among the Ig coats detected on tumor cells grown in vivo.