The present study aimed at confirming the observations that the proportion of aneuploid cells in blood culture increased with age due probably to the appearance of cells with an XO sex chromosome constitution. A total of 1, 700 cells from 17 females (8 aged and 9 young) were examined, and the difference in frequency of aneuploid cells between the aged and the young group was found to be statistically significant. Some consideration regarding the production of cells with 45 chromosomes are given.