[en] Three groups of adult female mice (S.w.), were exposed whole-body irradiation of 137-Cesium gamma rays at 2, 4, and 6 Gy/mn). For all groups, samples were taken at 2, 6, and 24 hours after irradiation and immediately fixed in a 1.4 % solution of glutaraldehyde containing 0.1 M sodium cacodylate buffer. Specimens were post-fixed in 1 % osmium tetroxide in the same buffer, then dehydrated in ethanol and 1, 2-epoxy-propane, impregnated in a mixture of propylene oxide and epon before being embedded in pure epon and polymerized at 60 deg C during 24 hours. Using a diamond knife and an ultra-microtome, ultrathin sections of approximately 50 nm thick were cut and deposited on copper grids. The sections were stained with uranyl acetate and lead citrate and studied with the electron microscopy (Philips 300). Early ultrastructural lesions were characterized by condensation of chromatin, convolution and fragmentation of nuclei. Clarification of cytoplasm and mitochondrial alterations were observed in a number of thymocytes depending on the radiation dose although the ultrastructure of the other ones remains normal. These lesions increased according to the length of the post-irradiation period. Clarifications of the cytoplasm with important mitochondrial lesions, as well as modifications of the cytoplasmic membrane were observed 24 hours after irradiation at 2 Gy and a number of polymorphonuclear cells and macrophages were observed in the thymus. Most often a single macrophage contains several apoptotic thymocytes, up to eight. The same alterations were observed in the other irradiated groups, but the extent of damage was much more important. In an other experimentation at 6 Gy the earliest ultrastructural lesions of apoptosis have been observed 15 minutes after the irradiation. (authors)