Tissue-incorporated radionuclides impart radiation energy over extended periods of time depending on their effective half-lives. The capacity of vitamin A dissolved in soybean oil to protect against the biological effects caused by internal radionuclides is investigated. The radiochemicals examined are DNA-binding 125IdU8 cytoplasmically localized H125IPDM and the α- particle emitter 210Po citrate. As in our previous studies, spermatogenesis in mice is used as the experimental model and spermatogonial cell survival is the biological end point. Surprisingly, soybean oil itself provides substantial and equal protection against the Auger effect of 125IdU, which is comparable to a high-LET radiation effect, as well as the low-LET effects of H125IPDM, the dose modification factors (DMFs) being 3.6 ± 0.9 (SEM) and 3.4 ± 0.9, respectively. The protection afforded by the oil against the effects of 5.3 MeV α particles emitted by 210Po is also significant (DMF = 2.2 ± 0.4). The presence of vitamin A in the oil further enhanced the radioprotection against the effect of 125IdU (DMF = 4.8 ± 1.3) and H125IPDM (DMF = 5.1 ± 0.6); however, no enhancement is provided against the effects of α particles. These interesting results with soybean oil and vitamin A, together with data on the subcellular distribution of the protectors, provide clues regarding the mechanistic aspects of the protection. In addition, the data for vitamin A reaffirm our earlier conclusion that the mechanism by which DNA-bound Auger emitters impart biological damage is primarily indirect in nature.