We measured the blood coagulability in pregnancy. Factor XIII, that showed unusual variations in these experiment, was investigated for its biological significance. The results are summarized as follows: 1) Blood coagulation factors XII, IX, VIII, VII and X, prothrombin and plasminogen showed a increasing tendency as the course of pregnancy. These was not very much change in factor V and plasmin activity during pregnancy. Factor XI showed a decreasing tendency. The further dramatic variation appeared in each coagulation factor at the onset of labor. 2) With regards to the factor XIII, XIII-S a carrier protein was inclined to increase from the value of 94.0 +/- 16.0% before pregnancy and reached the value of 109 + 16.3% at the last stage of pregnancy, while XIII-A an active site showed a decreasing tendency from 97.5 + 17.5% before pregnancy to 67.0 + 12.0%. 3) In placenta homogenate XIII-A was observed, but XIII-S absent. In retro-placental blood XIII-S was observed, but no XIII-A was seen. No significant difference between uterine venous blood and maternal peripheral blood was recognized. 4) The placental localization of XIII-A was investigated by the methods of fluorescent antibody technique and enzyme labeled antibody technique. XIII-A appeared in syncitiotrophoblast, but was not found in stroma, vessel wall and fetal side. In regards to decidua region its presence was not confirmed.