A foamy virus present in human nasopharyngeal carcinoma tissue was studied for a number of biological properties, including range of cellular susceptibility, growth curve, evolution of cytopathic effect in relation to cellular fusion and intracellular viral distribution, reverse transcriptase activity, and buoyant density. The virus was also studied immunologically and found to be closely related to the chimpanzee foamy viruses, particularly simian foamy virus type 6, with which it shares common antigens in complement-fixing, fluorescent, and neutralizing antibody tests. In view of this close immunological relationship and the failure to find antibody to the human isolate in sera from more than 250 humans, including 50 patients with nasopharyngeal carcinoma and Burkitt's lymphoma, it is suggested that the isolate is not a human representative of the foamy virus group but rather a variant strain of chimpanzee foamy virus.