Arg-Pro-Pro-Gly-Phe (RPPGF, BK[1-5]), is a stable metabolite of the peptide hormone bradykinin. Considering the short half-life of bradykinin (BK, approximately 15 secs), RPPGF has been used as a marker for BK's endogenous generation. A lack of a radioiodinated RPPGF has precluded the development of a radioimmunoassay for this peptide. The present study describes a two-step reaction that allows for the incorporation of 125I into the aromatic ring of the phenylalanine of RPPGF. This radioiodinated analog is recognized by an antibody to RPPGF, demonstrating its utility for the development of a radioimmunoassay for measurements of RPPGF, a stable metabolic product of bradykinin.