To explore the effects of interleukin-2 (IL-2) treatment in a vaccination protocol in the elderly, we administered low-dose rIL-2 to a group of aged subjects before primary tetanus toxoid immunization. A specific antibody response was detectable in the serum of 6/8 treated individuals after primary immunization, but in only 2/6 untreated controls; following antigenic boosting, specific antibody levels remained relatively unchanged in all the seroconverters. The data were confirmed by studying the ability to produce tetanus-specific antibodies in vitro, and by isoelectrofocusing analysis of serum anti-tetanus antibodies; this latter study showed a more restricted clonal response to the immunogen in untreated individuals. On the other hand, the study of the in vitro proliferative response to tetanus toxoid did not evidence clear differences between the two groups. On the whole, these data seem to indicate that a short-term rIL-2 treatment is able to potentiate the antibody response to tetanus toxoid, and may be a useful tool to improve humoral responses to vaccines in aged subjects.