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A total of 1544 sera from 28 diverse and mainly isolated populations were examined for HI antibody to BK virus. A few extremely isolated populations were found with negligible or absent exposure to the virus, but in most populations, remote or cosmopolitan, antibody appeared in increasing prevalence during early childhood and remained stable throughout adult life. Antibody acquisition and prevalence rates in individual families reflected that of the general population. Examined for HI antibody to JC virus were 393 sera from 9 of the 28 populations. Age acquisition and prevalence rates of antibody were similar to those of BK virus, but experience with the 2 viruses was found to occur independently in several population groups, i.e., high exposure to BK with low exposure to JC, or vice-versa. Examined for neurtralizing antibody to SV40 were 151 sera with and without BK HI antibody in individuals from several primitive populations. SV40 antibody, mainly in low titer, occurred in 35% of the BK-positive group, but only 5% of the BK-negative group, suggesting that infection with BK or a closely related virus is responsible for antibody directed against SV40 in most humans unexposed to known vaccine or monkey sources of SV40 infection.