Although pUlmonary fibrin deposition and coagulation abnormalities have been observed in acute lung injury In humans, their role In the pathogenesis of pulmonary disorders Is unclear. In order to gain further Insights Into the role of the coagulation in lung Injury, weexamined the relationship between procoagulant activity In bronchoalveolar lavage (BAL) fluids and the evolution of bleomycin-indUCed lung Injury In marmosets. The BAL procoagulant activity was Increased at 1, 2, and 4 wk after bleomycin challenge compared with that in control subjects, and It was capable of shortening the recalcification times of plasmas deficient in factor VII and factor Viii but not in factor X. This profile suggested the presence in BALof an activator of factor X. Activation of purified human factor X by BAL was demonstrated by measuring the amidolytic activity of the generated factor Xaon Its N-benzoyl-L-isoleucyl L-giutamyl-glycyl-L-argenine-p-nitroaniiide substrate. Factor