It is known that bony ingrowth implants without intimate apposition results in diminished ingrowth. In this study the use of bone morphogenetic protein (BMP) and cancellous bone graft to augment ingrowth in interface voids is investigated in the canine. Eight CoCr surface replacement hip implants of chamfered cylinder design with beaded CoCr porous layer were implanted in four adult canines and followed from 23 to 56 days. Prior to implantation, cylindrical defects 4mm diameter by 3mm deep were created in the femoral head and acetabular surfaces and filled with 10mg of BMP in one hip and cancellous bone graft in the contralateral hip. Animals were followed with serial radiographs to sacrifice after which microradiographs of ground sections and histologic specimens were analyzed. All defects were 100% filled with viable new bone without differences in density, organization, or ingrowth in the BMP and cancellous specimens. Femoral specimens uniformly showed unexpected seating gaps which were bridged to varying degrees with viable new organized bone. In five femoral cases seating defects of 2mm or more were completely bridged with ingrowth at the porous layer. In one acetabular case a 6mm gap was 90% bridged. No correlation of results was noted with time to sacrifice. It was concluded that new bone present in the seating gaps was most likely due to spillage of material from the cylindrical defects. Although the amount of ingrowth required for durable fixation is unknown, incongruencies of implant fit are common. As BMP showed no discernible advantage over cancellous graft in results of this study we advocate liberal use of readily available cancellous graft (from reamings or femoral head) to fill interface voids.