Gamma-ray- and fission neutron-induced micronuclei in PHA stimulated and unstimulated human lymphocytes.

Two groups of normal human blood cells, one stimulated with phytohaemagglutinin (PHA) for 24 hr (G1-S phase of the cell cycle) and one unstimulated (G0 phase), were irradiated with 60Co gamma rays or 252Cf radiation. A comparison of radiation-induced micronucleus frequencies showed that the high-dose-rate gamma rays were more effective in inducing micronuclei than low-dose-rate gamma rays. In the cells exposed to low-dose-rate irradiation, there was little difference between the frequency of micronuclei in the G0 phase and the G1-S phase. However, cells in the G1-S phase were more sensitive than G0-phase cells to high-dose-rate gamma rays. The relative biological effectiveness of 252Cf neutron irradiation measured in micronucleus assays was consistent with the value obtained for the lethal effect of 252Cf on cultured cells.