In addition to their well-known roles in hemostasis, fibrinogen (Fg) and fibrin (Fn) have been implicated in a number of other physiological and pathophysiological events. One of these involves the fibroproliferative response after acute lung injury, which is the focus of the current study. Mice with a total Fg deficiency (FG^−/−) were generated by breeding heterozygous (FG^+/−) pairs, each of which contained an allele with a targeted deletion of its Fg-γ-chain gene. The resulting FG^−/− animals did not possess detectable plasma Fg. FG^−/− mice were then used to assess the roles of Fg and Fn in a bleomycin-induced acute lung injury model. Intratracheal administration of bleomycin in wild-type and FG^−/− mice resulted in equivalent deposition of interstitial collagen and fibrotic lesions at days 7 and 14 after administration. This indicates that Fg and/or Fn are not essential for the development of bleomycin-induced pulmonary fibrosis.