The shielding of the abdominal region during X-irradiation is important for two reasons: following higher irradiation doses it prevents the development of the gastrointestinal syndrome; it prevents the development of the lethal form of the marrow syndrome by shielding a portion of the spinal column bone marrow and the spleen. The dose reduction factor in so irradiated mice, compared to whole-body irradiated mice, was 2.6. At 15 min after irradiation with the dose of 10 Gy, the number of spleen colony-forming cells (CFU-S) was decreased in the femoral bone marrow of both the group receiving whole-body irradiation and the group having the abdominal region shielded during irradiation. This great decrease persisted in the femoral bone marrow of both groups during the 5 h after irradiation. At 48 h after whole-body irradiation, the number of CFU-S in the femoral bone marrow of the unshielded animals decreased almost to zero values, and most of the mice were dead by 120 h after irradiation. In the group which had the abdominal region shielded, a statistically significant increase in the number of CFU-S in the femur was observed at 48 h when compared to the number of CFU-S in the femur at 5 h after irradiation. At 120 h, the number of CFU-S in so irradiated mice was at the level of the unirradiated group.(ABSTRACT TRUNCATED AT 250 WORDS)