Antibody responses to JCV viral and T antigens were determined in owl monkeys following infection with JCV. Antibody to JCV virion antigen was present in all inoculated monkeys, arising within one months. These antibodies gradually diminished in all nontumor-bearing monkeys and in 10/13 tumor-bearing monkeys over the ensuing 12 months. Antibody to T antigen became evident later in most inoculated monkeys but had a marked variable course and did not correlate well with tumor production. The antibody patterns suggest that JCV is basically nonpermissive in owl monkeys and that the viral genome is retained in a prolonged persistent stage before progressing rapidly into a tumor.