Summary Immunodiffusion tests have demonstrated that 6C 3HED ascites tumor cells, after freezing and storage over liquid nitrogen, lost one of their major antigenic components. The medium of the frozen cell samples retained the dissociable antigen. The type of eryoprotective agent used made no difference to the loss of antigen. By ether-alcohol treatment, the dissociable antigen could be extracted from untreated tumor cells. Antiserum produced in rabbits against this extract agglutinated and yielded bright fluorescence around the cell surface of untreated cells, but had no effect on cells which had been frozen in glycerol, DMSO, or myo-inositol. These observations suggest that surface antigen is involved. No correlation between the antigenic loss and the loss of host specificity could be demonstrated, however.