Summary Two hundred and ninety-nine sera from wild animals of various species (bats, bear, foxes, raccoons, meadow mice, cottontails, and deer) were tested by hemagglutination-inhibition or neutralization in mice, or both, for antibody to group A (Eastern encephalomyelitis and Western encephalomyelitis) and group B (Powassan [POW] and St. Louis encephalitis) arboviruses. The sera were obtained from animals trapped in 15 counties in New York State. Group B antibody was found in animals from 12 counties. In one, Schuyler County, up to 52% of 100 fox and raccoon sera tested had antibody for group B viruses, predominantly for POW, and up to 11% had reactivity for group A. Sera from 411 persons from institutions for the mentally retarded and from 31 patients with suspected encephalitis were examined by one or both of the test methods. Arbovirus antibody was infrequent in the human sera. Six persons (1.3%) showed some antibody for group A, and 28 (6%) for group B; more were detected by hemagglutination-inhibition tests than by neutralization. Neutralizing and hemagglutinstion-inhibiting activity for POW virus was found in 2 sera from a young boy in Albany County whose history did not indicate an apparent infection.