Retention of radiolabeled proteins in experimental thrombi formed in rabbits following extraction in 5.0 M urea. Rabbits were co-injected with 131I-fibrinogen and 125I-proteins identified in Figure 6 and jugular vein clots were formed as described in "Methods" Clots were γ-counted at the close of the experiment, and then extracted overnight with 5.0 M urea, the supernatant aspirated, and the clots recounted. The fraction of the total injected radiolabeled protein dose found in the clot after urea extraction is shown for 131I-fibrinogen, identified by the co-injected 125I protein on the × axis in (A) and for 125I-labeled proteins in (B). Differences between 131I-fibrinogen among the three groups in (A) are not statistically significant; the extent of statistically significant differences between groups is shown for the data sets linked by horizontal lines in (B). The mean ± SD of 12 determinations is shown.