Argonaute proteins interact with small RNAs that guide them to complementary target RNAs, thus leading to inhibition of gene expression. Some but not all Argonaute proteins are endonucleases and can cleave the complementary target RNA. Here, we have mutated inactive human Ago1 and Ago3 and generated catalytic Argonaute proteins. We find that two short sequence elements at the N terminus are important for activity. In addition, PIWI-domain mutations in Ago1 may misarrange the catalytic center. Our work helps in understanding of the structural requirements that make an Argonaute protein an active endonucleolytic enzyme.