A 40-year-old male biochemist had an intermittent fever over a period of nine days. The only significant symptoms were malaise, chilliness, sweating, headache, myalgia, and minimal stiffness of the neck. The results of laboratory tests differed but little from the normal, except that the urinary excretion of phenolsulfonphthalein was consistently delayed. Since the patient had been working with viruses for four years, inoculations were made into monkey kidney tissue cultures and Hela tissue cultures, using specimens of blood, urine, and cerebrospinal fluid as well as swabs from throat and anus. Intraperitoneal and intracerebral inoculations were also carried out in mice. The specimens of urine were prepared for inoculation by a concentration technique. The organism was identified as a Coxsackie B virus, and it appeared in a urine specimen collected on the fourth day of illness. It was also found at various times in the blood, cerebrospinal fluid, and pharyngeal and anal swabbings. Its appearance in the urine suggests viral infection of the kidney as the probable cause of the retention of phenolsulfonphthalein.