In agreement with its known binding locality remote from the C5a cleavage site (6, 8), OmCI does not prevent thrombin mediated cleavage of C5a from C5 (Fig. 10A), therefore the inhibitory activity of bOmCI-LTB4 seen in IC-ALI is likely due to inhibition of C5 activation solely by the C5 convertase and not by non-C proteases released by activated alveolar macrophages (18, 19). This evidence concerns the gene C5AR1 and acute respiratory distress syndrome.