A simple and reproducible micro tissue culture assay has been devised in RK13 and LLC-RK1 rabbit kidney cells for the titration and neutralization of rubella virus. In this "one-step" assay all virus and serum dilutions were prepared with spiral loops in disposable microplates and tissue cultures suspended in medium 199 and 3% horse or fetal calf serum were added to the microcups simultaneously. Micro tissue cultures were kept in a humidified incubator (36 °C) under a constant pressure of 5% CO2 for 8 days and were read microscopically for viral cytopathic changes on the seventh and eighth day. The microneutralization test performed in LLC-RK1 cell cultures was shown to be a reliable method for the detection of small amounts of rubella antibodies in human sera.The micro assay may be also applied to the virus titration of live, attenuated rubella vaccines.