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.