We previously demonstrated that around 6 weeks of age, most of the interferon-gamma (IFN-gamma)-/- C57BL/6 mice began to lose morphogenesis-derived hairs in their dorsal and occipital areas and that hair regrowth in the alopecic site was induced by intraperitoneal (i.p.) injection of IFN-gamma and allogeneic Meth A cells. Here, we explored the IFN-gamma mRNA expression in the cells infiltrating into allograft in IFN-gamma(+)/(+) mice by RT-PCR and adoptively transferred specific antigen-minus infiltrates into IFN-gamma-/- mice to assess the hair regrowth inducibility. IFN- gamma mRNA was expressed in the infiltrates on days 3-8 after allografting, with a peak on day 3 or 4, and CD4(+) and F4/80(+) cells were the major producers of IFN-gamma. All infiltrates on day 3 induced hair regrowth, whereas those on days 0-2 or 4-8 were ineffective or partially effective, respectively. The removal of F4/80(+) macrophages from all infiltrates failed to induce hair regrowth, whereas the removal of Ly-6C(+) macrophages rather accelerated the hair regrowth. These results showed that F4/80(+), Ly-6C(+), and CD4(+) and F4/80(+) cells were stimulatory, inhibitory, and IFN-gamma-producing cells, respectively, in the regulation of hair regrowth.