The basic hand of the helix of the nucleocapsid of haemagglutinating virus (Japan) was determined to be left-handed from observation of serrated-smooth asymmetry in tilted specimens examined in the electron microscope according to Finch's (1972) technique. Absolute determination of the helical hand was made by comparison with a tilted model of the virus nucleocapsid and by comparing this with the helical sense of the tobacco mosaic virus particle determined by the same method. The left-handed sense of the helix in the nucleocapsid was recognized by examination of cases showing opened-out turns of the helices. Under these conditions the far side of the particle was found to be contrasted predominantly. These conditions are discussed.