Contractile activity of bradykinin fragments, from dipeptide to octapeptide, was examined with guinea pig ileum (Magnus method) and the activity was compared with that of bradykinin. Some of the peptides having arginine on the C-terminal side had a comparatively strong activity, the strongest being XXVII whose activity was 4.3% (molar ratio)of that of bradykinin. Of the peptides without arginine, XXVIII showed the strongest activity. It was found that phenylalanine in 5-and 8-positions of bradykinin was important in the activity of the fragments. Smaller fragments of peptide fractions in XXVIII were also found to have an activity. Based on these observations, it was assumed that the active fragment of bradykinin lies in H-Phe-Ser-Pro-Phe-Arg-OH (XXVII). None of the bradykinin fragments antagonised the activity of bradykinin, and XXI and XXXI potentiated the bradykinin activity. Of these fragments, XXXVII, XXXVIII, and XXXIX did not have any ileum contractile activity or anti-bradykinin activity. The fragment XL, formed by substitution of serine with β-alanine had an activity 0.018% of bradykinin, the activity having decreased to about 1/200 of that of the corresponding peptide (XXVII).