Treatment with PEG-CAT alone or combination with PEG-SOD inhibits sensitivity of B9 cells to low-dose ionizing radiation. Cells were pretreated with PEG alone, PEG-SOD and/or PEG-CAT (200 U/ml individually or 100 U/ml each in combination) for 4 h prior to irradiation. Panel A: B1 cells. Panel B: B9 cells. Bars represent means of three separate experiments. Error bars represent ±1 SEM of 12–18 dishes from three different experiments. *Significantly different from 10-cGy irradiated B1 cells, P < 0.001; **significantly different from 10-cGy irradiated, “PEG alone” treated B9 cells, P < 0.001; #significantly different from 10-cGy irradiated, “PEG alone” treated B9 cells, P < 0.05.