Human cord blood and adult red blood cells (rbc) and pigeon rbc were compared for sensitivity to rubella virus hemagglutination by a new method, consisting of the direct counting of free and aggregate red blood cells by using an hemocytometer chamber. With this method, more reliable than the "pattern test", hemagglutination is quantitated by the "aggregation index" (A.I.). Pigeon rbc were more sensitive than human rbc; cord blood and trypsinized adult human rbc showed the same sensitivity. However the kinetics of hemagglutination, evaluated by A.I. after different incubation times, were essentially the same with the three rbc types.