The advent of antibiotics has made the diagnosis of the abscess in the patient postoperatively a most difficult task at times. The orthodox signs and symptoms are camouflaged by the administered antibiotics, so that frequently only a mild low-grade fever and a sense of ill-being are the only clinical manifestations of a hidden abscess. Patients have been observed who have gone steadily downhill after an operation with no localizing signs or symptoms, and then at autopsy the hidden abscess was encountered. In attempting to devise methods by which a hidden abscess might be diagnosed, advantage was taken of the findings of Whipple1that when a sterile abscess was formed, the pus contained a great deal of plasma protein. This study was accordingly undertaken to note if serum albumin tagged with radioactive iodine, if given intravenously, would localize in the abscess. Methods Six mongrel dogs were used as the animals