We have compared the gamma-irradiation survival of G0 peripheral blood lymphocytes from 18 neonatal cord blood samples in a cloning assay with results from 21 controls (age range 1-65 years and consisting of 20 adults and one child). Using mean inactivation dose as the discriminating parameter, the cord blood cells showed a significantly greater radiosensitivity (mean inactivation dose for pooled data = 1.54 Gy) than the normal controls (mean inactivation dose for pooled data = 1.90 Gy, p less than 0.001). These results confirm and extend earlier work suggesting that T-lymphocytes in newborn children are more radiosensitive than normal controls, and this may have implications for the radiation protection of the unborn child.