The modification of the standard rubella haemagglutination-inhibition (HAI) test using trypsin-treated human group O erythrocytes instead of chick erythrocytes was evaluated. In a comparative study we found that, of 816 samples tested by both methods, the titres of 807 (98.9%) sera were in close agreement within an acceptable twofold difference. Trypsin-treated human group O erythrocytes usually provided titres that were twofold higher than those obtained with chick erythrocytes. In general, a very good correlation between the two methods was obtained. Data are presented that emphasise the importance of trypsin treatment of human group O erythrocytes before use in the HAI method. Furthermore, we found that trypsin-treated human group O erythrocytes can be stored for periods of up to 30 days and used in the HAI test without any appreciable loss of sensitivity or specificity. Moreover, we replaced chick erythrocytes with trypsin-treated human group O erythrocytes in the sucrose density gradient/HAI method used for the detection of rubella virus-specific IgM and found it to be a very satisfactory method. In view of these findings we recommend that trypsin-treated human group O erythrocytes should replace chick erythrocytes in the standard rubella HAI test since the former provided not only a more sensitive, more economic, and less time-consuming method but also a technique which is as specific as that using chick erythrocytes.