This study quantified the constitutive and interferon-gamma (IFN-gamma) stimulated expression of MHC class I (HLA-ABC and beta 2 microglobulin) and class II antigens (HLA-DR, -DP, -DQ) on normal and malignant oral keratinocytes using radioimmunoassay and immunocytochemical techniques. Normal keratinocytes and three of four malignant cell lines (H103, H157, H314) expressed MHC class I antigens constitutively; IFN-gamma increased MHC class I expression with significant changes in normals, H157 and H314. Normal keratinocytes expressed significantly more constitutive MHC class I antigens than H103 and H157 and significantly more IFN-gamma stimulated MHC class I antigens than H103, H157 and H314. MHC class II antigens predominantly were not expressed constitutively on normals, H103 and H157 but, in H314, HLA-DR, -DP and -DQ antigens were demonstrated on 35, 11 and 5 per cent of cells, respectively, and resulted in a non-coordinated pattern of expression (HLA-DR greater than -DP = -DQ). IFN-gamma induced HLA-DR on normals, H103 and H157, whilst HLA-DP and -DQ remained undetectable. In H314, IFN-gamma enhanced HLA-DR, -DP and -DQ (significant increase of HLA-DQ) but the interrelationship between these antigens was maintained (HLA-DR greater than -DP = -DQ). Normal keratinocytes expressed significantly more IFN-gamma stimulated HLA-DR than H103 and H157 but significantly less HLA-DR than H314 under similar experimental conditions. One oral malignant cell line (H191) did not express MHC class I and MHC class II antigens either constitutively or in response to IFN-gamma. The results demonstrate aberrant patterns of MHC expression (absence, enhanced, diminished) in the different malignant oral keratinocyte cell lines.