A segment of tibia 4.5 centimetres long was removed from one hind limb of fifteen dogs. It was then replaced and the main vasculature was restored by a microsurgical technique. In eight controls the segment was replaced without such restoration. In two-thirds of the former cases the microvascular reconstruction was successful; the rate of infection was found to be reduced, bone union was guaranteed and the rate of union accelerated. Success or failure of the reconstruction was clearly demonstrated in five cases by early bone scanning using technetium-labelled polyphosphate.