With use of the immunoadsorption technique the protein antigenically related to pregnancy-specific beta1-glycoprotein (SP1), was isolated from urine of pregnant women. The physical and chemical properties of this protein were determined and compared with those of SP1. The antigen isolated from urine has a smaller molecular weight and is immunochemically only partly identical with SP1 found in pregnancy serum respectively in the placenta. The protein occuring in urine seems to be a split product of SP1 and is supposedly formed by an enzymatic degradation of the native proteins. The concentration of the protein in urine of pregnant women throughout gravidity was quantitated immunochemically using a modified Laurell-technique. During the first and second trimester of pregnancy values ranged from 0,0-0,4 mg/100 ml. In the last trimester there was an increased in the concentration of urine-SP1 with values ranging from 0,0-3,0 mg/100 ml and a mean concentration of about 1 mg/100 ml.