Keratinocytes exposed to interferon (IFN)-gamma synthesize major histocompatibility complex class II antigens both in vivo and in vitro; however, the expression of class II accessory genes has not yet been investigated. In this study, we examined the capacity of normal human keratinocytes activated with IFN-gamma to express HLA-DR, HLA-DM, and invariant chain genes as well as two major transcription regulatory genes, class II transactivator and RFX5. Cultured keratinocytes were shown to synthesize low levels of DM alpha, invariant chain p33, and RFX5 transcription factor. Upon treatment with IFN-gamma, expression of RFX5, DM alpha, and invariant chain p33 mRNA increased, whereas class II transactivator mRNA appeared de novo, followed by the expression of DR alpha, DMbeta, and invariant chain p41 genes. Western blot analysis showed that both p33 and p41 invariant chain forms and DM became detectable in keratinocytes after stimulation with IFN-gamma, with a higher p41/p33 ratio compared with Raji B cells. Finally, HLA-DR molecules present on IFN-alpha-treated keratinocytes were shown to be remarkably resistant to sodium dodecyl sulfate denaturation at room temperature, a feature that class II molecules acquire when their groove is properly loaded with peptide. These results suggest that human keratinocytes activated with IFN-gamma possess the biochemical requirements for the generation of functional class II peptide complexes.