Experimentally induced infection with high doses of Mycoplasma pulmonis results in acute pneumonia characterized by severe pulmonary hemorrhage, edema, and, often, death in C3H/HeN mice. To determine whether specific disease manifestations were associated with coagulopathy, we measured serum fibrin, fibrinogen degradation products, and plasma fibrinogen concentrations in C3H/HeN mice infected with high doses of a virulent strain of M. pulmonis. We also examined the lungs and other tissues from infected mice for the presence of intravascular fibrin clots and other lesions. Increased concentrations of fibrinogen degradation products indicated that coagulopathy occurs in acute M. pulmonis infection; however, intravascular fibrin clots were not present. Rather than decreasing, as might be expected during a consumptive coagulopathy, fibrinogen concentrations increased. The hyperfibrinogenemia probably is associated with an acute phase response to M. pulmonis infection.