Investigation of serum and tissue homogenates obtained from first, second and third trimester pregnancies, and from non-pregnant women, has provided further insight into the possible origin of the CA 125 antigen. Serum CA 125 levels were higher in the first trimester (median 53.6 U/ml, range 15.6-268.3 U/ml) than in non-pregnant women (median 19.3 U/ml, range 7.2-27.0 U/ml) and later in pregnancy (second trimester: median 18.5 U/ml, range 12.0-25.1 U/ml, third trimester: median 19.2 U/ml, range 16.8-43.8 U/ml) (P less than 0.05) but were two orders of magnitude less than in second trimester amniotic fluid (median 4825 U/ml, range 3200-9300 U/ml). Fetal serum CA 125 activity was consistently less than 20 U/ml. The highest tissue levels of CA 125 were detected in first trimester decidual homogenate (median 4547 U/100 mg, range 340.4-20 851 U/100 mg) and were greater than in non-pregnant endometrium (median 388 U/100 mg, range 100.9-3341 U/100 mg) (P less than 0.01) and term decidua (median 116 U/100 mg, range 32.7-449.9 U/100 mg) (P less than 0.01). These observations suggest that CA 125 is synthesized by normal endometrium and decidua and that increased CA 125 activity during pregnancy is of decidual origin.