Several serological studies of rubella virus (RV) infection were carried out on sera of 45 pregnant women having RV hemagglutination inhibition (HI) antibody titer more than 1:512 in the first trimester, including a follow-up study of these women's pregnancies. In order to detect RV-specific IgM antibody, various procedures were employed as follows: Complement fixation test (CF), HI test after treatment of the sera with Protein-A (ProA-HI), enzyme linked immunosorbent assay (ELISA) by indirect or sandwich procedures and several tests using fractionated sera by sucrose density gradient ultracentrifugation (SDG). Positive ratios found with the CF test and ProA-HI were 26.7% and 20.0%, respectively. In indirect ELISA, 4 cases were positive for RV-specific IgM, but all sera were negative in sandwich ELISA and SDG tests. Both RV isolation from urine of some infants and detection of high RV-specific IgM antibody in cord sera failed to be positive. Of 44 pregnancies, 4 cases resulted in spontaneous abortion and all others in normal delivery. These forty normal infants had no sign of congenital rubella syndrome, but one case among them showed polysyndactyly . The results obtained here seemed to indicate that sandwich ELISA is the most valuable and convenient method among the tested procedures for clinically determined diagnosis of recent RV infection in pregnant women having a high HI antibody titer.