IFN-γ has been reported to induce expression of HLA class II (DR, DQ, DP) antigens on cultured human dermal fibroblasts (FB) by stimulating the de novo transcription of the α and β chain genes of HLA-DR, -DQ, and -DP in these cells. We examined the relative nominal and alloantigen-presentation capacity of each HLA class II gene product on FB by using CD4-positive, TNP-specific T cell clones restricted by determinants on DR, DQ, or DP molecules, as well as allospecific, CD4-positive T cell clones recognizing DR-, DQ-, or DP-lymphocyte activating determinants. After IFN-γ exposure, FB strains used for antigen presentation displayed a high percentage of DR-positive cells and a much smaller percentage of DP-positive cells, but no detectable DQ-positive cells by immunofluorescent techniques. FB stimulator cells supported proliferative responses of two DR-allospecific T cell clones and one TNP-specific, DR-restricted clone, but not another TNP-specific, DR-restricted clone. Despite only modest DP expression, FB stimulated both a TNP-specific, DP-restricted clone and a DP-allospecific T cell line. However, IFN-γ treated FB failed to stimulate a TNP-specific, DQ-restricted clone and a DQ-allospecific clone. Our data indicate that IFN-γ differentially regulates expression of functional class II lymphocyte activating determinants on FB antigen-presenting cells and that FB may fail to support DQ-directed T cell responses due to insufficient expression of DQ molecules on the FB cell surface. However, the quantity of DR or DP expressed on FB did not directly correlate with their ability to support T cell responses, indicating that additional factors, such as differences in T cell clone activation requirements, contribute to the capacity of FB to present class II allo- and antigen-restricting epitopes.