Poliomyelitis virus was isolated from the blood of 6 of 33 individuals infected with a Type 1 strain. Of the 27 negative for viremia, 18 already had Type 1 antibodies. Type 1 virus was isolated from the throat swabs of all 6 who had viremia, and from the 5 rectal swabs tested. These specimens were all collected at the same time as the positive blood specimens. The clinical circumstances under which viremia was detected included the minor illness, the asymptomatic infection, and one instance in which the virus was isolated several days before the onset of a mild non-paralytic attack. In one instance, virus was isolated from a blood specimen which also contained Type 1 antibodies. The possible role of viremia in the pathogenesis of poliomyelitis is discussed.