Peripheral blood T lymphocyte subpopulations were measured, using a fluorescence-activated cell sorter, in fetal blood samples obtained either by cordocentesis (n = 118) or at elective caesarean section (n = 14). Both the numbers and percentages of the total T lymphocytes (CD3+) and T-helper lymphocytes (CD4+) increased exponentially with gestation from respective means of 46% (1.15 × 109/l) and 29% (0.70 × 109/l) at 16 weeks to a plateau of 75% (3.11 × 109/l) and 54% (2.10 × 109/l) at 34 weeks. Similarly, the number of suppressor/cytotoxic T lymphocytes (CD8+) increased linearly with gestation from a mean of 22% (0.55 × 109/l) at 16 weeks to 24% (0.96 × 109/l) at 40 weeks; there were no natural cytotoxic T lymphocytes (CD3+CD56+) in any of the fetal blood samples. The helper-to-suppressor T lymphocyte ratio (CD4/CD8) increased exponentially with gestation from a mean of 1.22 at 16 weeks to 2.57 at 28 weeks. The alterations in T lymphocyte subpopulations were accompanied by changes in the expression of CD45RA, L-selectin, CD25 and HLA-DR. These alterations in T lymphocyte subpopulations with gestation reflect the pattern of maturation and development of the fetal cell-mediated immune system.