Mice with polymicrobial sepsis develop anti-PF4/heparin antibodies. C57BL/6 mice underwent CASP, which caused polymicrobial sepsis, or were sham-operated. At 1, 3, 7, and 14 days after surgery, sera were tested for anti–mu-PF4/heparin IgM (A) and IgG (B) as described in “P4/heparin antibody assays.” Mice with polymicrobial sepsis (black bars) developed anti–mu-PF4/heparin IgM from day 3 and anti–mu-PF4/heparin IgG from day 14. Binding of IgM and IgG was reduced by excess heparin (100 IU/mL, striped bars). When mu-PF4 alone was coated (open bars), IgM antibodies showed minimal binding, but the IgG antibodies also reacted. Antibody titers of mice with CASP were always higher than those of sham-treated mice (gray bars). At each time point, at least 6 mice were assessed (days 1, 3, and 7: n = 6 each for CASP and sham-operated mice; day 14: n = 8 for CASP, n = 11 for sham). Data represent mean OD ± SD. All comparisons of OD values were performed between the OD values obtained at the same experimental day using as reference the values obtained when the sera of CASP mice where incubated with PF4/heparin complexes (black bars). *P < .05; **P < .01; ***P < .001.