In a series of 57 sera collected from poliomyelitis patients during the acute or subacute stage of their illness, 9(16%) gave strongly positive reactions, 5(9%) gave questionable reactions, and 43(75%) gave negative reactions when tested for the presence of hemagglutination-inhibitory substances against Columbia SK virus. All of 12 sera from patients convalescing from Japanese B encephalitis gave negative reactions. In a control group of 80 sera from healthy individuals 2(2.5%) gave positive reactions, 4(5%) gave questionable reactions, and 74(92.5%) gave negative reactions. Of 14 positive or questionable poliomyelitis sera, 5 contained significant neutralizing antibody levels against Columbia SK virus as detd. by mouse test.