The current study was conducted in an attempt to evaluate theeffect of Bio-Gen as a bone substitute in critical bony defects created inthe mandible of dogs as following:Clinically: by examining and observing the experimental animals and tocompare between test side and control side (without material placement).Radiologically: By observing bone formation through comparingbetween test and control sides.Microscopically: By observing and following up bone formation stagesat all bony defects in mandible whether testing side or control side.This experimental study was performed on 18 mongrel healthy dogsof 20 kgs body weight and average age 1 year old, the dogs were dividedinto six groups each contained three dogs, for each animal two operationswere performed in each right and left sides, where a critical bony defectof (29X 15X 6 mm) was performed. The right was the test side whereBio-Gen is applied while the left side was left without Bio-Genobliteration to be as a control side.The dogs were sacrificed in a predetermined periods as following,after one week, after two weeks, after three weeks, after four weeks, aftersix weeks, after eight weeks.The mandible was disarticulated and separated into two halves atthe symphysis. Both right and left sides were examined radiographicallythen prepared to be examined microscopically and the results were asfollowing:Clinical examination showed good healing of the animal injurieswithout early or late allergic reactions or cytotoxic reactions due to thepresence of the testing material (Bio-Gen) also there is no immunologicalreactions observed.Radiographic examination revealed that the test side where the materialplaced showed rapid bone growth more than the left side (control side).Histopathological examination showed that all the specimens takenfrom the right side which contains the testing material showed an increasein amount and rate of bone formation starting from the third week andthroughout the following periods than the other control side which is freefrom testing material.On the basis of the findengs and results of the current study thefollowing was concluded:1- Large bony defects of a critical size defect (20X15X6 mm) resultingfrom surgical of diseased tissues can be filled effectively and safely byBio-Gen material if it was surrounded by healthy bone.2- Large bony defects can be effectively obliterated with Bio-Gen withoutbiological membrane if this defect has supporting walls.3- Bio-Gen can be placed in extraction sockets preserving the width ofthe ridge for constructing of removable or fixed prothesis or implantplacements as Bio-Gen inhances bone repair and remodelling in fasterrate than the control animal.4- Bio-Gen is biocombatible in bone and soft tissue, bioactive andinhance bone formation.5- Bio-Gen has no side effects of allergy or poisoning action or cellularinjury.6- Further clinical studies about Bio-Gen use in bony defect repair arerecommended in human jaws to preserve the height and the width of thealveolar ridge for fixed and removable prosthesis and for dental implants where Bio-Gen stimulate bone formation and stimulate its reformation into lamellar bone.