The effects of low dose single whole-body irradiation on the specific hemolysin level in mice sera were studied in male Kunming strain mice, 18~22 g, aged 6~8 weeks. The mice were immunized intraperitoneally with sheep red blood cells (SRBC) after exposure to low dose Cobalt-60 γ-rays. The results showed that: (1) the levels of hemolysin in mice exposed to 2.5~10cGy were significantly higher than those of control (,p,<,0.05 or ,p,<,0.01) , and the maximum response occurred at 10cGy, so the optimum stimulating dose is 10cGy; (2) irradiation of 10cGy caused that the peak level of specific hemolysin in sera appeared earlier and significantly higher than that of control; (3) the hormesis of specific hemolysin induced by 10 cGy γ-rays could last 1 to 2 weeks.