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We previously demonstrated that around 6 weeks of age, most of the interferon-γ (IFN-γ)⁻/⁻ 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-γ and allogeneic Meth A cells. Here, we explored the IFN-γ mRNA expression in the cells infiltrating into allograft in IFN-γ⁺/⁺ mice by RT-PCR and adoptively transferred specific antigen-minus infiltrates into IFN-γ⁻/⁻ mice to assess the hair regrowth inducibility. IFN- γ 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-γ. 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-γ-producing cells, respectively, in the regulation of hair regrowth.