The percentage and the absolute number of T3, T4, T6, T8, and T10 lymphocytes of the peripheral blood were determined in 100 children (3 days to 10 years), 29 adults (18 to 59 years), and 30 elderly individuals (60 to 98 years). The percentage of the various T lymphocyte subsets and T4/T8 lymphocyte ratio for the children and elderly did not differ statistically from adults, except for the proportion of T4 lymphocytes which was higher in the subgroup with 3 days of age. The absolute number of total lymphocytes, T3 and T8 lymphocytes was increased in children up to the age of 6 years while the number of T4 lymphocytes was increased up to the age of 2 years. Increased numbers of T6 lymphocytes were detected in children from 1 to 2 years of age while the number of T10 lymphocytes was increased in children up to the age of 2 years. Finally there was no difference between the results observed in elderly individuals and adults.