SUMMARY The properties of infectious bronchitis virus strain Massachusetts were investigated. It was found that; the best dilution for IBV vaccine was 10. The peak titer reached after about 40 hours of inoculation. Concerning the predilection site of IBV multiplication was amino - allantoic fluide (AAF) where LD50 tỉter reached 10 /ml, while it was 10 and 10**/ml in both embryo and chorioallantoic membrane (CAM), respectively. ETER Dealing with the route of inoculation the highest titer was achived in allantoic sac ELD50 10° and 10 / ml, while CAM and yolk sac Y/S reached ELD50 and 10°/ ml, respectively. The results of thermostability of the IBV virus at 56 C revealed that the titer decreased from 9.16 to 4.6, 3.35, 2 and zero when the vials held for 5, 10, 30 and 45 minutes, respectively. The keeping quality of the allantoic fluid at + 4°C revealed that peak titer declined quickly at short time, and that kept at 37°C dropped sharply from 10%: 1oto 104°/ ml in 2 days. On the other hand freeze dried live IBV stored at -20°C did not show any significant loss in virus titer kept for more than one year. In freeze dried ampoules kept at + 4°C for 10 months, the reduction in virus titer was 1.56 Log while those kept at 37 C, the drop in virus titer reached 5. 16 Log in 24 days. The loss in virus titer inte ampoules at room temperature for about 3 months was 6.3 Log. Sie