In the 1949 epidemic in Easton, Pa., fecal samples were collected from 48 consecutive patients admitted to the hospital for poliomyelitis. From 15 of these patients Melnick and Kaplan1 of Yale University isolated both poliomyelitis and Coxsackie viruses. The question arose as to whether this meant a dual infection or merely a passive transfer of one virus through the alimentary tract. To answer this question they obtained serums from two typical paralytic patients on the third and fourth day of the disease and about 30 days later. An 18,000 r.p.m. supernate of a 1: 10 dilution of the patient's stool was mixed with an equal volume of his own diluted serum. After standing for two hours at room temperature, each serum-virus mixture was inoculated into three monkeys in a test for poliomyelitis virus and into at least eight newborn mice in a test for Coxsackie ("C") virus.