The capacity of paraformaldehyde-fixed human antigen-presenting cells (APC) to induce responses by autologous, freshly isolated peripheral blood T4 cells was examined and was compared with their ability to stimulate allogeneic T4 cell DNA synthesis. Fixation of glass-adherent cells (AC) with as little as 0.06% paraformaldehyde abolished leucine incorporation, whereas fixation with 0.75% paraformaldehyde caused death of greater than 98% of the AC. Control APC were able to take up and present the soluble antigens streptokinase-streptodornase (SK-SD), tetanus toxoid, or tuberculin-purified protein derivative to autologous Ia-depleted T4 cells. Fixation with greater than 0.06% paraformaldehyde eliminated such ability. When AC were incubated with antigen overnight and were then fixed, however, they were able to present nominal antigen to autologous T4 cells in a genetically restricted manner that was blocked by monoclonal antibodies directed against monomorphic determinants on class II major histocompatibility complex (MHC) molecules. Despite the ability to present nominal antigen, paraformaldehyde-fixed AC were unable to induce allogeneic T4 cell proliferation. Similar results were observed when non-T cells or spleen cells were used as stimulators. The inability of fixed APC to stimulate allogeneic T4 cell DNA synthesis was not reversed by increasing the number of fixed APC or by the addition of control AC autologous to the responding cells. Moreover, interleukins 1 and 2 either alone or in combination also failed to permit maximal T cell proliferation in response to fixed allogeneic APC. The differential effects of fixation on nominal antigen and alloantigen presentation could not be explained by the loss of membrane thymocyte stimulatory activity on fixed AC. These results indicate that antigen-bearing fixed APC are competent to stimulate proliferation by antigen-reactive T4 cells, but are deficient at inducing allogeneic T4 cell DNA synthesis. The differential sensitivity of these two Ia-restricted functions of APC to chemical denaturation (reductive methylation) by paraformaldehyde suggests that the allodeterminants and restriction elements for nominal antigen on MHC class II molecules can be functionally dissociated.