Aging has been associated with declines in somatotropin and IGF-I levels as well as declines in immune function. To determine the effects of age and whether ST administration could reverse immunosenescence in horses, eight young and eight aged female standardbred horses were given 10 mg/d recombinant equine somatotropin (eST) or vehicle buffer for 49 d. Plasma IGF-I concentrations in both age groups were higher in eST-treated animals (P < 0.001), and higher in young eST-treated mares than in aged eST-treated mares during wk 4 to 7 (P < 0.001). There was a trend toward lower monocyte and granulocyte numbers (P = 0.07) in mares treated with eST. Aged mares treated with eST had lower lymphocyte numbers (P < 0.005). The percentage of CD4+ lymphocytes was higher in aged mares (P < 0.001), and the percentage of CD8+ lymphocytes was higher in young mares (P < 0.01). Lymphocyte proliferation in response to concanavalin A, phytohemagglutinin, and pokeweed mitogen was not lower in aged mares (P = 0.17, 0.17, and 0.13 respectively). Aged mares treated with eST showed a lower peak primary antibody response to keyhole limpet hemocyanin (P < 0.05). Young mares treated with eST showed a higher peak primary antibody response to keyhole limpet hemocyanin (P < 0.05). Like other species, horses exhibit similar signs of age-related declines in various immune parameters, but those of aging were not reversed with eST treatment.