PAI-1 antigen and activity were determined in the CSS with increased rebleeding times, CSS-5, CSS-11, and CSS-17 (Figure 6), the heterosomic, CSS-5F1, CSS-17F1, and double hetersomic, CSS-5 × 17. CSS-5, CSS-17, CSS-5F1, and CSS-17F1 generally had reduced PAI-1 antigen when compared to B6 mice. The reduced PAI-1 antigen was a dominant trait due to the fact that the values were similar in the homosomic and heterosomic CSS-5 and CSS-17. The CSS-5 × CSS-17 double heterosomic strains had values similar to the B6 and A/J parent strains. PAI-1 activity was significantly reduced in the CSS-5 and CSS-17F1 mice, but again in the CSS-5 × CSS-17 strain the value for PAI-1 activity was restored to the value of the B6 and A/J parent strains. The PAI-1 antigen (5.4 ng/mL ± 0.5, n = 7) and activity (170 U/mL ± 20, n = 6) in the CSS-11 mice was not different than for B6 mice.