Experiment 1 involved genetically lean and obese gilts and barrows injected for 42 d with 0, 2, or 4 mg/d of porcine somatotropin (pST; n = 96). Blood was collected at 0, 6, and 24 h after the initial pST injection and thereafter every 2 wk. Experiment 2 involved lean and obese gilts and barrows (n = 48) that were administered implants that released 0, 2, or 4 mg/d of pST for 42 d. Blood was collected at d 0, 7, 14, 28, and 42 of the study. Thymic weights and a final blood sample were collected at the end of the 42-d trial. Thymic weights increased (P < .01) with dose of pST in injected and implanted animals. Thymosin beta 4 increased (P < .05) in a pST dose-dependent manner in injected and implanted animals, but concentrations were increased more in implanted animals. Concentrations of thymosin beta 4 decreased or reached a plateau in gilts but continued to increase in barrows to the end of the trial. Thymosin alpha 1 concentrations were increased (P < .01) in barrows compared with gilts, but the concentrations were not related to dose of pST. Concentrations of pST in injected animals increased (P < .01) within 6 h after injection and had returned to basal concentrations by 24 h after injection. Concentrations of pST in implanted animals were dose-dependent (P < .05) and remained increased throughout the trial. Concentrations of IGF-I and -II were pST-dependent (P < .01) and increased throughout the trial period. Outside of the pST dose relationships, temporal changes in thymosin beta 4 were not highly related to changes in pST, IGF-I, or IGF-II. The pST dose-related increases in thymic weights and thymosin beta 4 concentrations are consistent with pST stimulation of the immune function in conjunction with overall increases of growth and efficiency of live weight gain in swine.