This paper deals with the study of CEA levels in urine from healthy control subjects, of patients with bladder carcinoma or previous bladder carcinoma, and other groups of patients. In 53% of the patients with bladder carcinoma and without urinary infection, urinary CEA was increased. There were no false positive results. It is concluded that urinary CEA is a valuable aid in the detection of malignant tumour growth in the bladder. The results are compared with several tumour parameters, e.g. tumour size, stage of disease and histological parameters. Evidence is presented that CEA in infected urine is not produced by bacteria and that, like soluble vaginal excretory products, this substance has antigenic groupings in common with CEA from bladder tumours.