The effects of short-term thymic hormone administration on age-associated immune function were evaluated. Two groups of individuals greater than 65 years of age were treated for 30 days with thymic extracts (TP1) or placebo; before and after this treatment a panel of in vitro and in vivo parameters was determined according to a very rigorous experimental protocol. In most individuals, TP1 treatment was associated with an improvement in cutaneous delayed-type response to PPD. Moreover, an increase in a circulating T cell subpopulation bearing the CD45R surface antigen ("virgin" T cells), and in NK cell cytotoxic activity was also observed in some subjects. Finally, lymphocyte responsiveness to PHA tended to increase, while no effect on lymphocyte ability to produce IL-2 following mitogen stimulation was observed. These findings suggest that TP1 treatment may influence age-related alterations in immune function parameters in some subjects.