For the comparative study of protein patterns of the sera of pregnant women, the protein in sera of normal male subjects, non-pregnant women, pregnant women and women delivered of children were analyzed by using the methods of SDS/polyacrylamide gel electrophoresis, two-dimensional electrophoresis and amino acid analysis. The results were as follows: 1. When the protein patterns of sera in normal male ranging from 10, 000 to 110, 000 daltons were compared to non-pregnant women by SDS/polyacrylamide gel electrohporesis, their protein patterns were same each other numerically but bands 3(22, 000 dalton) and 6(39, 000) were less in male than in non-pregnant women quantitatively. When the protein patterns in the pregnant women in which serum were collected two week intervals were compared with non-pregnant women, there was increased or decreased in several bands quantitavely. The protein patterns of sera in pregnant women were compared with those of non-pregnant women; band 3(22, 000) showed similar patterns each other until the 16th week but the quantity of protein was decreased continously from the 18th week to the third trimester of pregnancy. Contary, bands 4(24, 000), 9(69, 000), 10(70, 000), 12(80, 000), 14(86, 000), 15(91, 000) and 16(94, 000) were gradually increased in quantity from the begining of gestation, and band 7(51, 000) was increased until the 32th week of gestation only but somewhat decreased after this time. The quantities of bands 12(80, 000), 15(91, 000) and 16(94, 000) were relatively increased when the protein patterns of delivered women were compared with those of the third trimester of pregnancy. Women who were dilivered female children showed more increase in bands 4(24, 000), 7(51, 000) and 10(70, 000)than one who were delivered male chilren. 2. When the protein patterns of sera in normal males were compared with those of nonpregnant women by two-dimensional electrophoresis, three spots of spot a group were not appeared in the males and the spot c group in the males was less than in the non-pregnant women. In the pregnant women, albumin was significantly decreased during the 10-12 week of gestation but recovered after these times. And spot f(70, 000) was decreased in the 10th week of gestation but increased from this time. 3. Glutamic acid, lysine, aspartic acid, leucine and valine in pregnant women were large in quantity while methionine, isoleucine and glycine were small in quantity by amino acid analysis. The total amino acids were increased remarkably in the second trimester of pregnancy but began to decrease in the third trimester of pregnancy. As mentioned, this present paper deat with that proteins which consist of maternal serum were increased with specific period in pregnancy and that the change of characteristic protein patterns were identified in the serum protein of each trimester in the pregnant women. And furthermore, the study should be preformed for the sex-identification of a fetus in the early pregnancy.