Fr. I from guinea-pig spermatozoa which had DF activity against rabbit spermatozoa showed inhibitory effect on respiration and motility of guinea-pig spermatozoa. Measurement of DF activity was attempted in mouse in vitro fertilization system and the potent inhibition of sperm penetration into ova by Fr. I was confirmed, while the addition of anti-Fr. I-immunoglobulin (anti-Fr. I-Ig.) caused disappearance of fertilization inhibitory activity of Fr. I. The DF activity of Fr. I was partially destroyed during incubation at 60° for 2 hours and completely destroyed at 80° for 2 hours. Alkaline and acid treatment of Fr. I caused no effect on the DF activity. The Fr. I did not decrease its DF activity by the digestion with β-amylase. From the enzymatic point of view the inhibition of corona radiata penetrating enzyme by Fr. I was noted.
