The characterizations of oxygen radicals released from alveolar macrophages (AM) of rats and related measurement conditions were observed with the technique of ESR in this experiment. The results were summarized as follows.1. Of PMA, BCG, ConA and cis-platinum, the first two stimulated AM to produce hydroxyl radical.2. AM stimulated by different agents had different kinetics of the release of the hydroxyl radical. The release peak from AM appeared at min. two after stimulating AM with PMA, with BCG the release peak delayed obviously.3. 5×10⁷AM/ml proved the fit concentration for trapping the signal of the hydroxyl radical from AM in this experiment. 4. There was a great influence of DETAPAC or EDTA on the intension of the signal.