Fluorescence microscopy was used for evaluation of new bone formation in 16 canine mandibles augmented with hydroxylapatite (HA) granules. Three fluorochromes were injected at different time intervals during therapeutic radiation treatment. Oxytetracycline, DCAF, and alizarin-complexone were given intravenously to mark the bone level at these times, respectively. Oxytetracycline, which defined the baseline of bone at implantation of HA, was detectable in 42% of animals that were irradiated and in no animal of the nonirradiated control group. The marker DCAF, designating levels of bone at the start of radiation, was demonstrated in 92% of irradiated animals, and in 75% of animals in the control group. The uptake of alizarin-complexone determined the level of bone found at the end of irradiation. This marker was demonstrated in 50% of the dogs irradiated and in 75% of the control dogs. Bony trabeculae were found between and at the surface of the HA granules. New generation of bone directly on the HA granule and in the surrounding haversian systems as part of normal bone turnover was demonstrated to take place more than 5 months after implantation of HA.