In this paper,a label-free,sensitive,rapid,simple and general colorimetric method was reported to monitor nuclease activities based on protamine-assisted aggregation of gold nanoparticles(AuNPs).Here,protamine,a linear polycation,was used as a medium for causing the aggregation of negatively charged gold nanoparticles(AuNPs)by electrostatic interaction,which resulting in the changes of surface plasmon resonance(SPR)absorption bands as well as color of AuNPs(Fig.1).