Short and long virus particles of soil-borne wheat mosaic virus (SBWMV) were purified by three cycles of differential centrifugation using sucrose density gradients after preliminary partial purification from leaf extracts.  It was shown by electron microscopy that the purified short-particle fraction contained no long particles, whereas the purified long-particle fraction was always contaminated with a few short particles.  Infectivity assay on Chenopodium quinoa indicated no infectivity with short particles, and some infectivity with long particles, whereas the infectivity appreciably increased when they were mixed.  The infectivity dilution curve of a mixture of purified long and short particles was of the double-hit type.  Particles of both short and long types were always found in individual local lesions examined by electron microscopy when C. quinoa was inoculated with a purified long-particle fraction.  It is likely that long particles and short particles are separately noninfectious, but they become infectious when they are mixed.  Serological properties of the short and the long particles were closely related.  Heterologous complementation tests using four isolates were performed with short- and long-particle fractions.  Heterologous interaction could be observed in all of 12 possible combinations.  When long particles of one strain were mixed with short particles of another, it was found that the short particles controlled particle length, serotype, and type of inclusion bodies.  The long particles controlled the infectivity on tobacco and virus concentration in inoculated leaves of spinach.  Both particles influenced symptom types in rye and Tetragonia expansa, although the effect of the long particles appeared to be much greater than that of the short particles.  The parent virus systems were obtained again from these bybrids by repeating complementation in the opposite direction.