We report experiments to test the hypothesis that the increased yield of dicentric chromosomes observed in human peripheral blood lymphocytes treated with X-rays during the G1 phase of their first cell cycle, as compared with the yield when the cells are treated in their G0 phase prior to phytohemagglutinin stimulation, is a manifestation of the recently-reported conversion of an inactive form of DNA polymerase alpha to its active form as the PHA-stimulated cells pass from G0 into G1 (Sylvia et al., 1988). The specific polymerase alpha inhibitor butylphenyl deoxyguanosine was used as an X-ray post-treatment. The results show that polymerase alpha is not involved.