Two clones, E2‐7.7 and E2‐10.50, derived from two macrophage(MΨ)hybridomas, E2‐7 and E2‐10, have been studied. The first clone, E2‐7.7, is Ia and Fc receptor (FcR) negative and manifests a strong antigen‐presenting capacity. When we pulsed its cells in vitro with keyhole limpet hemocyanin (KLH) antigen and injected them into syngeneic animals, we found that as small a dose as 103 cells initiated an immune response in vivo. On the other hand, antigen‐pulsed cells of the E2‐10.50 clone, which are Ia‐ and FcR, were almost incapable of triggering immunity, even when injected at a dose of 105 cells. Thus, the two clones differ in their immunogenic capacity (both cellular and humoral immunity). In experiments aimed at testing the stimulation in vitro of primed lymph node (LN) cells by antigen‐pulsed cells of these two hybridoma clones, we observed that E2‐7.7 stimulated the unfractionated population of LN cells and the LN‐derived population of T cells. The E2‐10.50 cells stimulated only the unfractionated population of LN cells, but not the T cell population. Subsequent tests indicated that the E2‐10.50 cells require an intermediate Ia accessory cell to present the antigen to the T lymphocytes. Analyzing the molecular structure of the MΨ hybridomas, we discovered that major histocompatibility complex (MHC) genes of the myeloma haplotype (H‐2d), and of the splenic MΨ used for fusion (H‐2k), which were not expressed in the parental myeloma or in the E2‐10.50, were expressed in the E2‐7.7. Thus, somatic cell fusion of MΨ resulted in the activation of suppressed genes of the myeloma partner. It appears that these antigens participate in controlling the immunogenic properties of the E2‐7.7 clone. Testing the effects of interferons on the MΨ hybridomas, we observed that interferon‐γ activated, at both the mRNA and the cell surface‐antigen levels, the expression of H‐2Dk, H‐2Kd and H‐2Dd in the E2‐10.50 cells, but not in the E2‐7.7. Consequently, interferon‐γ augmented significantly antigen presentation by E2‐10.50 but not by E2‐7.7 cells. These two hybridoma clones might represent two distinct subsets of normal MΨ, manifesting two different sets of functional properties. Having described earlier the E2‐10.20 clone that takes up, but does not present antigen, we may in fact be facing three types of MΨ hybridomas, representing three functional subpopulations of MΨ.