Relative biological effectiveness of ionizing radiation is determined by a number of factors, including a dose rate. Radiotherapy equipment employs low dose rates of up to a few Gy per minute. But very little is known about the biological effect of high and ultrahigh (≥ 10⁸ Gy/min) dose rate radiation. Our study aimed to investigate the apoptotic effect of ultrahigh gamma dose rates on human peripheral blood lymphocytes. Blood samples were collected from seemingly healthy donors. Lymphocytes were isolated by density gradient separation. Lymphocyte suspensions were irradiated with low-rate doses on the Rokus- AM gamma-ray machine for clinical use (Russia) and with 10⁸ Gy/s doses on the experimental pulse generators Angara-5-1 and Mir-M (Russia). Apoptosis was measured by flow cytometry using annexin V and propidium iodide double staining. We established that in comparison with low dose rates, ultrahigh gamma dose rates (with doses ranging from 1 to 6 Gy) induced significantly more pronounced apoptosis in peripheral blood lymphocytes (p < 0.05) with fewer necrotic cells. Total radiation-induced cell death did not differ significantly between the therapeutic gamma machine and the experimental pulse generators. Further research is needed to assess biological and medical significance of our findings.