To investigate the physiological characteristics of the corpus luteum (CL) of pregnancy, we raised a mAb, human corpus luteum (HCL)-4, against human luteal cells obtained from CL of pregnancy. The affinity-purified antigen from human CL of pregnancy or placenta using HCL-4 was a 61 kDa protein. The partial amino acid sequence of the antigenic protein was identical to that of human monoamine oxidase A (MAOA, EC1.4.3.4). MAOA has been shown to catabolize catecholamines that were reported to regulate luteal function in CL and vasoconstriction in various organs. Immunohistochemistry using HCL-4 mAb showed that MAOA was intensely expressed on large luteal cells and moderately expressed on small luteal cells in the CL of pregnancy. In the CL of menstrual cycle, MAOA was weakly detected on large luteal cells but not detected at all on small luteal cells. Western blotting analysis confirmed the high expression of MAOA in CL of pregnancy. Northern blot analysis also showed the expression of MAOA mRNA in human CL, and showed that its expression was higher in CL of pregnancy than in CL of menstrual cycle. The increased expression of MAOA in the CL of pregnancy suggests the contribution of MAOA to the function of the CL of pregnancy.