In two separate studies rabbits were fed orally with human and rabbit "picobirnaviruses". Polyacrylamide gel electrophoresis (PAGE) of nucleic acid extracted from faecal samples collected from inoculated rabbits revealed the presence of discrete equimolar bands, typical of picobirnaviruses, in several specimens. The genome profiles detected in both studies differed significantly from that of the inoculum suggesting that passage of the inoculated picobirnaviruses had not taken place and that the bands were a co-incidental finding. The presence of rabbit picobirnaviruses was confirmed by characterization of the genome bands, as dsRNA by enzyme digestion and by their co-sedimentation in caesium chloride (CsCl) gradients with 32 nm virus particles at a buoyant density of 1.39 g/ml. Picobirnavirus genome segments varied in size in a range between 2.3-2.6 kilo base pairs (kbp) and 1.6-1.9 kbp for the slow and fast migrating bands, respectively. Immune electron microscopy of the picobirnavirus particles revealed round or slightly hexagonal particles with a smooth surface and a mean diameter of 30.7 nm. In one rabbit, an immune response, temporally associated with picobirnaviruses excretion, was demonstrated by immune electron microscopy (IEM) supporting the view that picobirnaviruses may be vertebrate viruses. Two antigenically distinct picobirnavirus strains were defined by IEM.