CA125 and CA130 in various tissues were characterized using the monoclonal antibodies OC125, 130-22, and 145-9. CA125 and CA130 were eluted in the same fraction in 130-22 affinity chromatography. A sandwich immunoradiometric assay using these antibodies in combination was established. When the solid phase of the assay was changed from OC125 to 145-9, the amount of 125I-OC125 that reacted with CA125 in the amnion increased more than that in other samples. On SDS-PAGE, CA125 and CA130 epitopes in the amnion migrated to different positions. These results indicate that CA125 and CA130 are located on different subunits of the same glycoprotein, and that their synthesis is regulated differently in various tissues.