This study was to compare the degree of cell-mediated immunity induced by three commercial Mycoplasma hyopneumoniae bacterins using interferon-γ (IFN-γ) measurements, lymphocyte stimulation assays and delayed-type hypersensitivity tests. Serum IFN-γ levels were significantly elevated in all four vaccinated pig groups at 21 days post-vaccination (P<0.05). Lymphocytes isolated 21 days post-vaccination exhibited significantly more proliferation in response to M. hyopneumoniae than lymphocytes isolated 0 day pre-vaccination (P<0.05). Following intradermal injection of M. hyopneumoniae antigen at 14, 21 or 28 days post-vaccination, all pigs in the four vaccinated groups displayed skin reactions characterized by circumscribed, often erythematous nodules. Taken together, these demonstrate that all three commercial single-dose M. hyopneumoniae bacterins used in this study induce varying degrees of cell-mediated immunity.