Abstract Fever is the cardinal manifestation of infection and may be blunted in certain infected elderly individuals. It is known that elevated body temperature enhances both the inflammation response and immune function, resulting in increased host resistance to infection. Recently, it has been suggested that the elevation of body temperature and the activation of lymphocytes by IL-1 are interrelated host effects. However, the question of whether fever response in vivo is closely correlated to cell-mediated immune parameters is unknown. In this study, a well-defined murine model was used to study the relationships between aging, fever and cell-mediated immune response. Thus, measurements of rectal temperature changes were made in individual young (4–6 months) and old (26–27 months) BALB/c mice to determine their ability to respond to endogenous pyrogen (recombinant IL-1). Splenic cells from these animals were used to assess T- and B-cell proliferation, production of interleukin 1 (IL-1) and IL-2. The results revealed that the proliferative capacity and the IL-1 and IL-2 producing capacity of splenic cells from old mice were markedly decreased. However, aging did not significantly affect the mean febrile responses in old mice following rIL-1 injections. Finally, there was no significant correlation between in vivo fever responses and the immune parameters measured in vitro in both young and old mice.