Abstract:Atrain of leptospira stable L-forms,isolated from patient with cerebral obstruction and identified with immunofluorescence and immunoenzyme technic was used for studing its pathogenicity in laboratory animals. Guinea -pigs infected with this L-forms were taken as the experimental group,and with leptospira original forms as the control group. On the 2nd day after infection,fever occurred in animals of both 2 groups and organisms wereisolated from blood in 3/5 of the experimental group and 4/5 of the control group. All animals were sacrificed 2 months after infection,and blood were taken again for cultivation,organisms could still be isolated from 3 of the experimental group,but none from the control group. In histological examination,the pathological changes found were mainly interstitial inflammations of eyes, heart,skeletal muscles and lungs,narrowing of sinuses and small arteries of liver,hemorrhages in lungs and degeneration of the nerve cells in the anterior horn of spinal cord in borh 2 groups. Leptospira L-forms were also seen in the convoluted tubules of 1 animal of the controll group.These results indicate that the stable L-forms might persist in the blood much longer than the original forms.And the late complication of the leptospirosis might be caused by L-forms induced through the medical treatment and body immunity in certain individuals. Leptospira L-forms could also be excreted in the urine and become a source of infection that should not be neglected.