We have previously shown that Interferon-Inducible Protein-10 (IP-10), a cytokine chemotactic for CD4-positive lymphocytes, is overexpressed by lesional epidermal keratinocytes and probably accounts for the epidermotropism of cutaneous T-cell lymphoma (CTCL). The tax gene of human T-lymphotropic virus-I (HTLV-I) immortalizes CD4-positive lymphocytes, induces IFN-γ, and has been detected in patients with classical CTCL who are seronegative for HTLV-I. TNF-α is synergistic with IFN-γ for the induction of IP-10. We therefore decided to define the presence of tax, IFN-γ, TNF-α, and IP-10 in lesions of 19 adults with classical CTCL who were seronegative for HTLV-I. Lesional mRNAs for actin, TNF-α, IFN-γ, and tax were detected by reverse-transcriptase polymerase chain reaction (RT-PCR) amplification. In addition IP-10, TNF-α, and IFN-γ were detected and localized with immunocytochemistry of frozen sections. In agreement with previous observations IP-10 was overexpressed in lesional keratinocytes of all 19 patients. By RT-PCR, mRNA for IFN-γ was detected in lesions of 8, and for TNF-α in lesions of 13 patients. By immunocytochemistry, TNF-α was expressed by lesional keratinocytes in 10 of 13 tested patients, whereas IFN-γ was focally expressed by lesional lymphocytes and faintly by lesional keratinocytes in 9 of 13 tested patients, tax mRNA was not detected in lesions of any patient, but was easily detectable in cutaneous lesions or peripheral blood of control patients who were seropositive for HTLV-I. We conclude that TNF-α and IFN-γ may cause epidermotropism by inducing IP-10. However, the tax gene of HTLV-I does not appear to be involved in the pathogenesis of classical CTCL.