The fusion of thioglycollate-elicited peritoneal macrophages from the lipopolysaccharide (LPS)-nonresponsive C3H/HeJ mouse strain to a hypoxanthine phosphoribosyltransferase (HPRT)-negative variant of the murine macrophage cell line P388D1 has resulted in the derivation of eight hybrid clones following HAT selection. Propidium-Iodide staining followed by flow cytometry has demonstrated that the DNA content of the hybrids represents the sum of the parents. Codominant expression of class I antigens from both parental haplotypes is observed in the hybrids. While class II antigens are inducible following a 72-hr induction with gamma interferon-containing supernatants, the amount of each haplotype varies between clones. These hybrids demonstrate Fc-mediated erythrophagocytosis in contrast to P388D1. In distinction to the C3H/HeJ primary peritoneal-macrophage parent, LPS treatment of the hybrids results in the increased release of both interleukin-1 (IL-1) and cachectin/tumor necrosis factor (TNF) into culture supernatants. Therefore, cell fusion has resulted in the stable restoration of the LPS-responsive phenotype in C3H/HeJ macrophage hybrids. These macrophage hybrids should serve as useful models in understanding the regulation of macrophage effector functions in response to environmental stimuli.