The susceptibility of Giardia-free jirds (Meriones unguiculatus) and AJ-strain mice to oral inoculation with cysts from Swiss Giardia isolates was assessed. Giardia cysts of human, dog, cat, and sheep origin produced infections in jirds (4-10 weeks old). AJ-strain mice (6 weeks old) were insusceptible to Giardia from man, dog, and cat. The number of Giardia cysts inoculated per rodent ranged from 5000 to 12,600. The course of infection was studied in jirds experimentally infected with a Giardia isolate of cat origin. After administration of 6300 cysts to each of 16 jirds, 5 (31%) started to excrete cysts 4-5 days post inoculation (p.i.). Cyst excretion patterns were mostly intermittent, patency periods lasted 5-28 days, and average cyst numbers per g feces of individual jirds varied between 118,500 and 835,200 (maximal number: 3216700). The parasites were eliminated spontaneously in all 5 jirds. The Giardia isolate of cat origin was passaged 4 times in the jird. A Giardia isolate obtained from sheep was transferred in the cyst stage to jirds. Giardia trophozoites isolated from one of these jirds have been axenically cultivated in vitro in a modified, filter-sterilized TYI-S-33 culture medium and successfully cryopreserved. To our knowledge, this is the first report on axenic cultivation of Giardia of sheep origin. Giardia from cats could not be cultivated in vitro under the same conditions. The fact that Swiss Giardia isolates from humans and various domestic animals are not strictly host-specific is in agreement with similar findings from other areas and suggests a zoonotic character for giardiasis.(ABSTRACT TRUNCATED AT 250 WORDS)