['Factors influencing alloimmunization to transfused red blood cells (RBCs) are not well understood. Utilizing a murine model, we have recently reported that RBC alloimmunization is enhanced by recipient treatment with viral-like polyinosinic polycytidylic acid (poly(I:C)). To determine whether a different subtype of inflammation also enhances RBC alloimmunization, we investigated the effects of the bacterial endotoxin lipopolysaccharide (LPS) on alloimmunization.', 'Mice were treated with poly(I:C) or LPS; in select experiments, the precursor frequency of naïve antigen-specific CD4+ T cells was increased using T cells from T-cell receptor transgenic mice. Recipients were transfused with leukoreduced RBCs expressing the membrane-bound hen egg lysozyme (mHEL) antigen, and alloimmunization was measured by anti-HEL immunoglobulin G responses using enzyme-linked immunosorbent assay and flow cytometric cross-match. Costimulatory molecule expression was examined on antigen-presenting cells (APCs) by flow cytometry.', 'Increased expression of costimulatory molecules on APCs was seen after treatment with either poly(I:C) and LPS. In contrast to the enhancement of RBC alloimmunization observed after treatment with poly(I:C), LPS not only failed to enhance but also actively suppressed alloimmunization, even in the presence of increased mHEL-specific CD4+ T cells (p < 0.001 LPS vs. control).'] Do discrete Toll-like receptor agonists have differential effects on alloimmunization to transfused red blood cells?