Antisera prepared against extracts of human nephroblastomas (Wilms' tumors) allowed the characterization in these extracts, and in a few extracts of tumors of other organs, of a tumor-associated antigen. This antigen is different from all previously described antigens, and was named W antigen (W for Wilms). It does not seem to be a carcinoembryonic antigen, and hence it could be of viral origin.