D-Leu-Pro-Arg-Rh110-D-Pro acetate is a substrate for Factor Xa I (FXIa) with binding affinity. D-Leu-Pro-Arg-Rh110-D-Pro acetate consists of Rhodamine 110 linked to a peptide chain through a cleavable bond. Cleavable bond cleavage enhances fluorophore intensity. D-Leu-Pro-Arg-Rh110-D-Pro acetate can be used to detect FXIa activity.