1. Japanese third molars were divided into sample groups based on the donor's year of birth, and 90Sr level in each sample group of 10 teeth was determined to compare it with the previously reported data obtained from sample groups of 28 teeth.
2. Trace and detection-limit amounts of 90Sr were detected in the teeth from donors born in 1929-1931. The 90Sr level increased in the teeth from donors born in 1935, and reached a peak value of 66.9-72.8mBq/g Ca in the teeth from donors born in 1953. It decreased afer 1954 and reached 25.5mBq/g Ca in the teeth from donors born in 1970.
3. Compared with previous data using 28 teeth as a sample group, good accordance was observed in period showing detection-limit level of 90Sr, tendency of increase thereafter, peak period, radioactivity at peak period, tendency of decrease after 1953 and radioactivity after peak period.
4. These results clearly indicate that a small sample, that is, 10 teeth, is sufficient for the examination of annual change in the 90Sr level.