ABSTRACT Using biochemical, serological, and cytopathological evidence, we have confirmed that banana bract mosaic virus (BBrMV) is a distinct member of the family Potyviridae. Virions of a Philippine isolate of BBrMV were purified from field-infected banana cv. Cardaba. Particles were approximately 725-nm long, banded at a density equivalent to 1.29 to 1.31 g/ml in cesium chloride equilibrium gradients, and had an A(260/280) of 1.17. Yields of about 4 mg/kg were obtained from fresh or frozen leaf midrib or lamina tissue. Three major protein species with sizes of 31, 37, and 39 kDa were resolved from dissociated virions, and all reacted specifically with polyclonal antibodies to BBrMV. Infected leaf cells contained typical pinwheel inclusions. Virus-specific cDNA was amplified from field samples by reverse transcription-polymerase chain reaction (RT-PCR) assay using potyvirus degenerate primers. In plate-trapped antigen-enzyme-linked immunosorbent assay (ELISA), weak serological relationships were demonstrated between BBrMV and other members of the family Potyviridae, including abaca mosaic (AbaMV), dasheen mosaic, maize dwarf mosaic, sorghum mosaic, sugarcane mosaic, and wheat streak mosaic viruses. Despite similarities in the symptoms caused by the two viruses, AbaMV was serologically distinct from BBrMV and reacted only weakly, or not at all, with BBrMV antibodies in double-antibody sandwich (DAS)-ELISA. No cross reactions were observed when RT-PCR products from the two viruses were examined by Southern blot hybridization using BBrMV- and AbaMV-specific digoxigenin-labeled DNA probes. BBrMV was consistently associated with banana bract mosaic disease, as assessed by DAS-ELISA and Southern blot hybridization using DNA probes. The known geographical distribution of BBrMV was extended to include India (Kokkan disease) and Sri Lanka.