Despite massive immunization programs, influenza remains a major cause of morbidity and mortality for elderly people. This may occur because immune senescent recipients may respond to vaccination with inadequate antibody production. We measured antibody response to the trivalent 1983-1984 influenza vaccine in young and elderly volunteers and found a significantly reduced response in the latter. The age-associated decreased antibody production was also observed in lymphocyte cultures in which specific antiinfluenza antibody synthesis was measured. In these cultures, however, the addition of a thymic hormone preparation (either thymosin fraction 5 or thymosin alpha 1) was shown to enhance specific antibody synthesis to a greater extent in the cultures established from the elderly volunteers. If this in vitro observation of thymosin induced increased antibody production reflects what might occur in a clinical trial in which elderly subjects receive thymosin coincident with vaccine, greater protection against influenza infection may result.