The passive haemagglutination and bentonite flocculation tests were compared for detecting plant viruses, using antibody-sensitized tanned red cells or bentonite particles. Parsnip yellow fleck, raspberry ringspot, turnip yellow mosaic, narcissus mosaic, potato X and tobacco rattle viruses were detected by both tests in purified preparations and, where studied, in crude plant extracts. The highest antigen titres were obtained only when the red cells or bentonite particles were sensitized with optimal amounts of antibody; these had to be found experimentally because they differed between antisera and between the two tests. In comparative experiments the bentonite flocculation and passive haemagglutination tests were respectively about 2 to 5 and 100 to 125 times more sensitive than tube-precipitin tests for detecting the elongated viruses and about 10 to 20 and 500 to 600 times more sensitive for detecting the isometric viruses. The minimum concentrations of virus detected were 1.8 to 2.6 µg./ml. with the bentonite flocculation test and 0.04 to 0.24 µg./ml. with the passive haemagglutination test. Red cells treated with formalin before they were tanned and sensitized with antibody could be preserved at -14° for up to 8 weeks without loss of activity.