Factor XIII is composed of two distinct proteins, a and b subunit. The dimers of each subunit form a tetrameric complex of a2b2 in plasma. Two separate ELISA systems were developed to measure either factor XIII a or b subunit in plasma. Sensitivity of the ELISAs was 3.0% of factor XIII a and b subunit in normal plasma. The ELISA for factor XIII b subunit quantitate free b2 and a2b2 complex in a equimolar manner. In order to evaluate the changes of each factor XIII subunit in a state of hypercoagulation, 16 plasmas from proteins with disseminated intravascular coagulation syndrome (DIC) were examined. Factor XIII a and b subunit showed 56.5 +/- 30.5% and 63.4 +/- 22.5%, respectively in the DIC subjects compared to normal pooled plasma as 100%. The ratio of factor XIII a and b subunit (a/b ratio) were between 0.5 and 1.0 in DIC cases which improved after treatment. However, the ratios were less than 0.5 in the cases with DIC which deteriorated in spite of the treatment. These results suggest that the a/b ratio would indicate the prognosis of patients with DIC.