Fibrinogen knock-out mice (Fgn−/−) and heterozygotes (Fgn+/−) on C57BL/6J background were sensitized with two i.p. injections with OVA + alum on days 1 and 14 and then exposed to 3 daily inhalations of nebulized 1% OVA. Non-sensitized and sensitized littermates (Fgn+/+) were used as wild type controls. AHR was assessed 48 hrs after the last exposure to OVA. The control mice had a significantly elevated response to inhaled methacholine affecting all three parameters of respiratory mechanics (Rn, H and G). In contrast, Fgn−/− did not show a response different from wild type control mice. Fgn+/− had an intermediary response. n = 11, *p<0.05, **p<0.01, ****p<0.0001.