Rubella virus antibodies were measured in 300 sera from pregnant women visiting a maternity center by using a new, simple latex test, Rubalex. The results were compared with those obtained by using hemolysis in gel, hemagglutination inhibition, and radioimmunoassay. The sensitivity of the latex test was 100, 98.0, and 99.6% when compared with hemolysis in gel, hemagglutination inhibition, and radioimmunoassay, respectively. Its comparative specificity was 96.2, 95.7, and 90.7%, and the predictive value of a positive result was 99.2, 99.2, and 98%, respectively. When assayed with the British standard anti-rubella serum its sensitivity was 11 IU/ml. The latex test gave a positive result within 2 min, and 87% of the positive samples had already reacted after 1 min. The negative results remained as such for at least 8 min. No prozone effect was observed for sera with hemagglutination inhibition titers from 256 to 2,048. We concluded that the latex test, Rubalex, was readily applicable for measuring rubella immunity with a reaction time of 2 min in undiluted samples.