The radioprotective capacity of chlorophyllin was determined by measuring the reduction of gamma-ray-induced sister chromatid exchange (SCE) in murine spermatogonia in vivo. The results obtained in experiments using 100 and 200 μg of chlorophyllin per gram of body weight (bw) and irradiated either before or after BrdU incorporation, indicate that a chlorophyllin dose of 200 μg/g bw protects 100% against the induction of SCE by 0.75 Gy of gamm-rays and 100 μg/g bw protects less than 50%. Chlorophyllin per se did not have any effect on the SCE frequency.