Lectin binding in normal and irradiated embryonic mouse tissues on day 10 of gestation was studied by peroxidase techniques. Specific binding of Dolichos biflorus lectin (DBA) was detected in the mesodermal blood vessels and in the otic vesicles. The amount of DBA as well as that of soybean agglutinin (SBA) and peanut agglutinin (PNA) increased after exposure to low doses of radiation (0.25, 0.50 and 0.75 Gy). The modifying influence of ionizing radiation was observed in the pituitary region, in the otic vesicles and in the blood vessel endothelium. The greatest effect appeared in the pituitary region at 0.75 Gy, while in the otic vesicles it appeared at 0.50 Gy. A dose-effect relationship was established for the DBA lectin affinity of the vascular endothelium. In comparison to DBA, SBA and PNA displayed more extensive staining after irradiation. The reactivity of these lectins appeared especially pronounced on the blood vessels within the central nervous system and in the luminal surface of the ependymal cells. It is of interest that maximal binding for PNA was observed at 0.25 Gy and for SBA at 0.50 Gy at the junctions between neuroepithelial cells.