H-2d thyroids cultured in oxygen at normal atmospheric pressure (suboptimal culture) were grafted into H-2b mice. Some of these tissues were cultured with recombinant mouse gamma-interferon (rIFN), and they expressed high levels of major histocompatibility complex antigens before grafting. Three weeks later, no difference in the rate of rejection of MHC-induced grafts was observed as compared with uninduced tissues (50% of each group). Fresh (uncultured) grafts were uniformly rejected in less than 2 weeks. Also, H-2d thyroids, freed of donor leukocytes by preculture in hyperbaric oxygen and more than 1 year parking in normal H-2b recipients, were incubated with and without rIFN, and then grafted into normal H-2b mice; 100% acceptance was observed in both groups regardless of the expression of allo-MHC molecules on thyroid cells. In another set of experiments, using grafts with a single antigenic difference at the MHC locus (bm1 into B6), graft rejection was observed only when the recipients were immunized with donor spleen cells and fresh tissues were implanted. In the same immune recipients, cultured and MHC-induced thyroids grafted in the opposite kidney were, in general, not rejected. These results demonstrated that the expression of allo-MHC molecules on graft cells was not by itself sufficient to engender tissue immunogenicity. This supports our previous hypothesis that the main effect of tissue culture is the inactivation of passenger leukocytes. MHC antigens appear to be immunogenic only when properly presented by these cells.