Abstract: Small, polybasic peptides are disclosed that are effective as furin inhibitors, e.g. hexa- to nona-peptides having L-Arg or L-Lys in most positions. Removing the peptide terminating groups can improve inhibition of furin. High inhibition was seen in a series of non-amidated and non-acetylated polyarginines. The most potent inhibitor identified to date, nona-L-arginine, had a Ki against furin of 40 nM. Non-acetylated, poly-D-arginine-derived molecules are preferred furin inhibitors for therapeutic uses, such as inhibiting certain bacterial infections, viral infections, and cancers. Due to their relatively small size, these peptides should be non-immunogenic. These peptides are efficiently transported across cell membranes.