[Objective]To investigate the effect of coagulation of intervertebral disc nucleus pulposus(IDNP) by alum.[Method]In vitro,20 canine intervertebral discs(I D) were divided into 4 groups,and immersed into 2.5%,5%,10% alum solution and normal saline respectively.Another 16 I D also randomized into 4 groups and were injected into I D with corresponding solution.In vivo,6 canine ID were grouped into 4:A.blank control;B.one injection of alum;C.two points injection of alum;D.injection of N.S.Experimental dogs were sacrificed and disc samples were taken at 3 days,2 wks,and 1 month post-injection.Gross observation,histological exam by microscopy and scan electron microscopy(SEM) and histochemical test were performed.[Result]NS did not coagulate the IDNP in vivo and in vitro,while in vivo,coagulation was seen around the injection points,two points with two coagulation blocks,and grew with time culminating on one month after injection,but there was no coagulation appeared after alum injection.In vitro,histological study showed aboundant collagenous fibers gathered in coagulation lock representing manifold function of mesenchyme.[Conclusion]The alum can make the IDNP to be coagulated in vivo,the mechanism of coagulation is the collagenous fiber proliferation and fibrosis due to manifold function of mesenchyme.