Present study was undertaken to find when is right time for vaccination against Measles, Mumps and Rubella and what is the seropositive conversion rate after those vacinations. For this purpose, sera from 106 infants and children adimitted in Prediatric Department of Won Kwang University Hospital, Iri, Chonbuk, Korea were divided into 3 groups, such as (1) Vaccination group with definite information when it was given, (2) Unknown group whether vaccination was given or not, (3) Not vaccinated group. They were tested of IgG and IgM antibodies against Measles, Mumps and Rubella using Enzyme Immunoassay method and the following results were obtained. 1. Infants below 6 month of age showed to have IgG antibodies which seemed to have been transferred from mother in 87.8%(29/33) for Measles, 78.8%(26/33) for Mumps and 39.4%(13/13) for Rubella. And they showed IgM antibodies which are thought to have been produced by recent infection in 24.2%(8/33) for Measles, 48.5%(16/33) for Mumps and 9.1%(3/33) for Rubella. 2. Positivity of antibody IgG against Rubella was observed remarkably lower than it is against Measles and Mumps being only 39.4%(13/33) in $0{\sim}5$ month, 30.8%(8/26) in $6{\sim}11$ months, 30%(3/10) in $12{\sim}14$ months and 62.9%(22/35) in $18{\sim}36$ months of age. 3. ${\Delta}OD's$ of IgG and IgM antibodies against Measles were observed increasing with age being 0.444, 0.220 in $0{\sim}5$ months, 0.326, 0.134 in $6{\sim}11$ months, 0.581, 0.140 in $12{\sim}14$ months, 0.512, 0.000 in $15{\sim}17$ months and 0.887, 0.278 in $18{\sim}36$ months of age, respectively. 4. ${\Delta}OD's$ of IgG and IgM antibodies against Mumps were observed increasing with age being 0.427, 0.340 in $0{\sim}5$ months, 0.400, 0.249 in $6{\sim}11$ months, 0.694, 0.314 in $12{\sim}14$ months, 0.539, 0.165 in $15{\sim}17$ months and 0.854, 0.350 in $18{\sim}36$ months of age, respectively. 5. Vaccination for Measles, Mumps and Rubella is generally to start at 15 months of age in Korea, by which age their antibodies are found to exsist in more than 80% of tested samples. So, it seems to be very reasonable to start the vaccination schedule at earlier age than it does currently. 6. From the present study, it seems to have been clearly confirmed that Enzyme Immunoassay method is a reliable method with good reproducibility for mass survey of IgG and IgM antibodies against Measles, Mumps and Rubella in infants and children.