A hemadsorption (HAd) reaction was demonstrated in rubella-infected cell cultures using erythrocytes of 1-day-old chickens, adult chickens, pigeons, geese, sheep or bovines. HAd occurred at 4°C, room temperature or 37°C, and at a pH of 6.2 or 7.4. BHK-21 cells were the most satisfactory for demonstrating HAd; the reaction could be demonstrated only in heavily-infected cultures of other cell types. Infected BHK-21 cell cultures generally had to attain infectivity titers of 10−3.5 to 10−4.0 InD50 per 0.1 ml in order for HAd to be demonstrable. The HAd reaction could be used to detect unneutralized virus in neutralizing antibody assays, and the inhibition of HAd in infected cell cultures was a very sensitive method for assay of rubella antibodies. A procedure was developed for identification of rubella virus isolates by HAd inhibition.