We have studied effects of long-term, low-dose growth hormone therapy on the immune function and life expectancy of Balb/c mice. Sixty male Balb/c mice were aged up to the time when they started showing signs of senescence and causal death (deaths started when they became 17 months old). The aged mice were divided into two groups of 26 mice each. One group received growth hormone (30 micrograms/mouse) subcutaneously twice a week for 13 weeks. The control group received an equal volume of saline for the same period. During this treatment period, 16 control mice died (61%) whereas only 2 of the hormone-treated mice died (7%). Four mice from each group were killed and immunological functions of splenocytes were evaluated. Hormone-treated mice had higher stimulation indices for pokeweed mitogen but not for Concanavalin-A. Total IgG production was decreased but IL-1, IL-2 and TNF production was increased. After a lag period of 4 weeks, growth hormone therapy was continued for another 6 weeks. One of the growth hormone treated mice died while the control group no longer existed. Splenocyte functions of the growth hormone treated mice were compared to those of young mice. The results showed no significant difference between cytokine production (IL-1, IL-2, TNF and IgG) in the young and the hormone treated groups. Stimulation induced by concanavalin-A and pokeweed mitogen however, was higher in the young group than the old group. The mortality curve obtained suggests that long-term low-dose growth hormone treatment prolongs life expectancy.