Observed rate constants, kobs, were determined under pseudo-first-order conditions at 10°C, ionic strength 0.15 and pH 7.4 in the low (A) and high (B) concentration ranges, as described in Experimental Procedures. Each kobs value represents the average of 16-37 fluorescence traces. ●, Native antithrombin; ○, Latent antithrombin; □, Cleaved antithrombin.