In a previous series of studies, we had shown that the constitutive Ia expression in an immunoselected Ia-human B cell variant, RJ 2.2.5, could be restored by somatic cell hybridization with mouse B cells. These experiments allowed us to show the existence of a transacting activator factor(s) operating across species barriers and encoded by the aIr-1 locus located on mouse chromosome 16. The aim of the present study was to investigate whether the B cell constitutive Ia expression and the inducible Ia expression, as seen in macrophages treated with IFN-gamma, are controlled by similar intracellular factors. To this purpose, we constructed an interspecies somatic cell hybrid between the human Ia-RJ 2.2.5 B cells and the mouse Ia-P388 D1 macrophage cells. These murine cells transiently express Ia antigens when incubated with IFN-gamma. Our results show that RJ 2.2.5 X P388 D1 cell hybrids do not express either human or mouse class II gene products. Treatment with human recombinant IFN-gamma did not modify the MHC phenotype of either the hybrid cells or the human parental cells. On the other hand, treatment of the hybrid cells with murine recombinant IFN-gamma resulted in de novo expression of mouse Ia mRNA and corresponding cell surface antigens without, however, reinduction of the human class II-positive phenotype. Furthermore, treatment with the mouse lymphokine significantly increased the levels of human HLA class I mRNA and corresponding cell surface antigens in the hybrid cells, further reinforcing the notion of the existence of non-species-specific secondary mediators generated after receptor-ligand interaction in the IFN-gamma system. Together, these results indicate that in macrophages, the intracellular events taking place after binding of IFN-gamma with its own receptor and leading to the expression of a class II-positive phenotype do not operate via an activation of the aIr-1 locus and/or its products. Thus, at least in our experimental system, we can firmly establish a first, relevant distinction between constitutive and inducible class II gene expression. This difference, dictated by the specific differentiation program of each cell type, may be relevant for the understanding of the function of class II gene products.