Infections with the avirulent (A7/74) strain of Semliki Forest virus which causes primary demyelination of the central nervous system in mice have been studied further in nude athymic (nu/nu) mice and their immunocompetent (nu/+) litter mates to measure the production of immunoglobulins. This has been done by radial immunodiffusion and enzyme-linked immunosorbent assays. Half the nude mice examined were able to synthesize specific IgG but at levels 1,000-fold lower than their nu/+ littermates. The majority of nude mice reconstituted with spleen cells from nu/+ mice 1 day before infection with virus were able to synthesize specific IgG nearly as well as the nu/+ animals.