The effect of high-energy (660 MeV) proton irradiation at the phasotron accelerator in FLASH mode (80 Gy/s) compared with the standard proton exposure power of 3.0 Gy/min was studied. When irradiated in two modes at doses of 1.0 and 1.5 Gy, the induction of cytogenetic damage in bone marrow cells and the state of lymphoid organs (thymus and spleen) were evaluated; survival under total in vivo irradiation of mice was analyzed at doses of 7.0 and 8.0 Gy. The growth rate of a model tumor under ex vivo irradiation was determined at doses of 40 and 60 Gy. It has been shown that irradiation of animals in the FLASH mode at a dose of 1.5 Gy protected the proliferative activity of the spleen and also led to a decrease in cytogenetic damage in bone marrow erythrocytes according to the micronucleus test compared with the standard irradiation mode at a dose of 1.5 Gy, that is, a milder effect of the FLASH mode dose was observed. However, irradiation of mice in FLASH mode at high doses (7.0 and 8.0 Gy) led to earlier death of animals compared to the standard irradiation regime. A tumor node formed with further growth only after FLASH irradiation of a suspension of Ehrlich ascites carcinoma at a dose of 40 Gy; in all other groups a tumor was not formed.