Soluble fibrin monomer complexes have been determined in approximately 500 obstetric patients by protamine sulfate precipitation, as a test for intravascular coagulation. The incidence of positive fibrin monomer was less than 1% in 139 samples drawn during normal pregnancy. In confirmed abruptio placentae, 84% of samples were positive, but other sources of antepartum bleeding were negative. Positive results were obtained in 24% of samples from patients with a dead fetus, in 19% of patients with preeclampsia, and in 18% with sepsis. In samples drawn from patients between 3 and 48 hours after injection of hypertonic saline for second trimester abortion, 33% were positive but only 3% were positive after administration of prostaglandins. The test for intravascular coagulation is simple and rapidly carried out. The results correlated well with the clinical condition of patients with disseminated intravascular coagulation. However, the test is usually negative in patients with thromboembolic phenomena.