Rheumatoid arthritis patients have antibodies in their serum directed against the perinuclear factor, a protein component present in keratohyalin granules in the cytoplasm of human buccal mucosa cells. The anti-perinuclear factor (APF) can only be detected by an indirect immunofluorescence test performed on fresh buccal mucosa cells from 'selected donors'. To obtain a more reliable antigen source and to gain more insight into the origin and nature of the perinuclear factor we attempted to culture perinuclear factor-containing buccal mucosa cells. Here we describe the successful culturing of such cells, which, however, did not contain keratohyalin granules nor the perinuclear factor. By adding the phorbol ester 12-o-tetradecanoylphorbol-13-acetate (TPA) we were able to induce keratohyalin granules in both cultured primary buccal mucosa cells and a squamous carcinoma cell line of the cheek (SqCC/Y1). These induced keratohyalin granules do contain the protein profilaggrin, which in vivo, in fresh buccal mucosa cells, co-localizes with the perinuclear factor. However, we were not able to demonstrate the presence of the perinuclear factor, not even after induction of terminal differentiation of the cultured cells nor after Epstein-Barr virus infection. Our results suggest that the perinuclear factor, in contrast to profilaggrin, is not an integral component of buccal mucosa cells.