A syncytium-forming (foamy) virus was isolated from a spider monkey brain cell culture. Cytopathic effect was observed both in the brain culture and in human embryonic kidney cells. Neutralizing antibody was present in the sera of the spider monkey from whom the isolation was made. The virus was inhibited by 5-bromo-2-deoxyuridine (20 mug/ml), contained a ribonucleic acid-dependent deoxyribonucleic acid polymerase, and had an infectivity peak at 1.15 g/cm(3) in a sucrose density gradient. The virus passed through a 220-nm but not a 100-nm membrane filter, was chloroform sensitive, and was inactivated at 56 C in 30 min. Hemagglutinating and hemadsorption activity was not noted with a variety of erythrocytes. The virion was spherical, formed in the cytoplasm, and was 105 to 115 nm in diameter. Ring-shaped nucleoids, 45 to 50 nm in diameter, were associated with tubular profiles. The virus was not neutralized by sera prepared against known viruses, including simian foamy virus types 1 through 7, Mason-Pfizer monkey virus, and bovine syncytial and measles viruses. Sera from a rabbit hyperimmunized with the isolate and sera from 19 spider monkeys had neutralizing antibody to the isolate; however, these sera did not cross-react with simian foamy virus types 1 through 7. Neutralizing antibody to the isolate was not detected in sera from 16 humans, 9 rhesus monkeys, and 10 chimpanzees.