Lazaroids (2 I-aminosteroids) are a novel class of compounds specifically designed to inhibit the iron-dependent lipid peroxidation associated with reperfusion injury. In the present study, we compared the effect of several lazaroids on radiationinduced apoptosis in explanted rat thymocytes and cultured murine lymphoma cells irradiated in vitro. An assay for the DNA fragmentation specific for apoptosis indicated that the lazaroids U-74500A, U-75412E, and U-74389F when dissolved in ethanol protected rat thymocytes and that U-75412E when dissolved in 10% cyclodextrin protected lymphoma cells. Additional experiments were conducted to ascertain which of the mechanisms associated with lazaroids were responsible for the inhibition of apoptosis. Other, nonsteroid inhibitors of lipid peroxidation-cysteamine, Desferal, and a-tocopherol-displayed no ability to inhibit apoptosis, suggesting that the steroid moiety of the lazaroid molecule was the active component. This was confirmed by showing that the nonglucocorticoid steroid testosterone inhibited radiation-induced apoptosis in these cell systems. Thus, lazaroids may inhibit apoptosis by interfering with specific functions of the cell membrane that are required for the manifestation of apoptosis and not by the inhibition of lipid peroxidation. We conclude that lazaroids may be useful agents for blocking radiation injury in normal tissues, but this will require confirmation in animal systems.