A total of 480 serum samples from donors including 384 children up to 10 years of age were examined by the hemagglutination-inhibition (HI) test for the rates of prevalence and age of acquisition of HI antibodies against JC virus and BK virus. Among 136 serum samples from various age groups, there were five (4%) with no detectable antibodies against BK or JC virus, 75 (55%) with antibodies against both viruses, 41 (30.1%) with antibodies against only BK virus and 26 (19%) with antibodies against only JC virus. The prevalence of antibodies against JC and BK viruses was 70.5% and 80.8%, respectively, and the mean HI titers (4 x 2n,n greater than or equal to 1) were 4.90 and 4.30. About 50% of the children had acquired antibodies against BK virus by 3 years of age and against JC virus by 6 years of age. These results indicate that dual latent infections with both viruses are common, although independent infections with either virus are predominant in the human population.