Objective To find out the pathogenicity of bacterial L-forms.Methods β-glucuronidase activities of both E.coli and their L-forms were determined with modified Fishman's method. The endotoxin content in 3 kinds of Gram negtive bacteria and their L-forms were measured with the limulus lysate test. Under ultrasonography, percutaneous transhepatic cholecystic punctures were performed on 2 groups of 10 healthy dogs. E.coli and their L-forms were respectively injected into the gallbladders of the 2 groups. 48 hours later, the gallbladder bile was obtained through puncture and cultured. After 6 months, all the animals were killed. Their bile was cultured and the gallbladders were pathologically analysed.Results Endotoxin existed in L-forms of Gram negtive bacteria (about 1/3-1/2 of the original forms). L-forms of E.coli also produced β-glucuronidase (58.67% activity of the original forms). A half of the E.coli which had stayed in the canine gallbladders for 48 hours have been transformed into L-forms by the action of bile and other factors. The L-forms existed in the gallbladder bile longer than 6 months. The gallbladders showed chronic infections: mild atrophy of mucosa, white blood cell and lymph cell infiltration in submucosa and slight fiberosis of the gallbladder walls. Bacterial L-forms were found in mucosa cells.Conclusions Bacterial L-forms can also produce pathogenical materials, but the amount is less than that of the original bacterial forms. Both bacterial forms and L-forms can lead to chronic infection of the canine gallbladders.