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Large bony defects often show a delayed healing and have an increasing risk of infection. Several materials are used for the coverage of large defects. These materials must be biocompatible, easy to use, and must have an appropriate stability to present a mechanical hindrance. Aim of this study was to investigate two different biodegradable membranes for defect coverage in a sheep model. Round cranial defects (1.5 cm diameter) were created in sheep. Six different treatments were investigated: defects without membrane, defects covered with a poly(D,L-lactide) or with a 70/30 poly(L/D,L-lactide) membrane and all defects with or without spongiosa filling. The sheep were sacrificed 12 or 24 weeks postoperatively. Bone formation in the defects was quantified by computer-assisted measurements of the area of the residual defect on CT radiographs. Histomorphometry and host-tissue response were evaluated by light microscopy. The biocompatibility was investigated by analyzing the amount of osteoclasts and foreign body cells. Both membranes served as a mechanical hindrance to prevent the prolapse of soft tissue into the defect. The biocompatibility test revealed no differences in the amount and distribution of osteoclasts at the two investigated time points and between the investigated groups. No negative effect on the tissue regeneration was detectable between the investigated groups related to the type of membrane, but a foreign body reaction around the two membrane types was observed. In the membrane-covered defects, the spongiosa showed a progressing remodeling to the native bony structure of the cranium. The groups without spongiosa partly revealed new bone formation, without complete bridging in any group or at any time point. Comparing the 12 and 24 weeks groups, an increased bone formation was detectable at the later time point. In conclusion, the results of the present in vivo study reveal a good biocompatibility and prevention of soft tissue prolapse of the two used membranes without differences between the membranes. An enhanced remodeling of the spongiosa into native bony structures under the membranes was detectable, but no osteopromoting effect was observed due to the membranes.