The carbohydrate antigen CA 50 is expressed in the epithelial tissue during the process of neoplastic transformation; i.e., transitional cell bladder carcinoma. The present study evaluated the biological behaviour of the CA 50 antigen in malignant superficial, deep localized and disseminated bladder tumors. One hundred subjects were entered into the study: 30 carefully selected healthy subjects comprised the first group and 70 patients with a diagnosed transitional cell bladder carcinoma comprised the second group. The serum CA 50 antigen was determined by immunofluorometric assay (Delfia CA 50 kit). Our results indicate that the carbohydrate CA 50 antigen can be utilized as a prognostic marker in patients with malignant bladder tumors. The serum antigen levels were higher for the more undifferentiated tumors and those in the advanced stages.