Summary: Neutralizing antibody to rubella virus was measured in tissue culture by a constant virusvarying serum test. Careful control of virus dose was the most important factor for obtaining reproducible results. Antibody titers were increased by diluting sera with unheated antibody-free rabbit serum; this enhancing effect was abolished by heating. With the standard test, rubella virus strains were shown to be antigenically similar, and different from several other human viruses. The test reliably differentiated between antibody positive and negative sera, and with comparable virus doses showed reproducibility of antibody titer similar to other neutralization tests. The immune status of military recruits was correctly predicted by presence or absence of antibody. Antibody appearance after clinical or subclinical infection was prompt and persisted for years.