Levels of CA125 and SCC antigen in amniotic fluids were examined in 26 cases. All samples showed high levels of CA125 and SCC antigen. Statistically significant decreases in CA125 (p < 0.05) and increases in SCC antigen (p < 0.01) from the second to the third trimesters were recognized. We focused on amnion cells as likely sources of the 2 tumor markers in amniotic fluid. Analysis of in vitro culture of amnion cells and amniotic membranes revealed an accumulation of CA125 and no accumulation of SCC antigen in the culture supernatant. A Northern blot analysis using a cDNA probe of SCC antigen ensured that there was no mRNA expression of SCC antigen in the amnion, the cord or the placenta. It is likely that the amnion is a major source of CA125 in amniotic fluid, and that the fetus is the origin of SCC antigen.