An attempt was made to measure the antibody to rubella virus in human sera by ELISA by calculating the relative potency by the method of the parallel line assay. As a standard serum, adult serum 8 months after rubella infection was used. Sample sera were collected from 42 infants before MMR vaccination and 8-10 weeks after the vaccination and 48 adults three years or more after rubella vaccination. In addition, three infant sera with rubella 6-19 days after onset of rash and three gamma immunoglobulin products were used. The dose response curve of the standard serum and that of the sample serum showed the lineality and the regression line of the sample serum was parallel to that of the standard serum. And the relative potency of sample sera was successfully calculated. The good correlation was observed between the rubella antibody titer by hemagglutination inhibition test and that measured by ELISA. The coefficients of variance of relative potency obtained from four repeated measurements for two weeks showed from 15% to 25% in four sera of HI high titer. The reproducibility of parallel line assay was also satisfactory.