A sensitive, specific radioimmunoassay for kinins has been developed, which is able to detect 1.5 pg bradykinin or 3 pg lysyl-bradykinin (Lys-bradykinin). 50% displacement in the standard curve was obtained with 10 pg bradykinin or 15 pg Lys-bradykinin in 0.6-ml incubates. The antisera, raised against bradykinin, recognized well Lys-bradykinin and methionyl-lysyl-bradykinin (Met-lys-Bradykinin), but cross-reacted 0.4% or less with bradykinin fragments. Kininogen cross-reacted only 0.2%. The radioimmunoassay and kininogen from several species were used in the measurement of human and rat urinary kallikrein activity. The peptide generated by hydrolysis of the substrates by rat or human urines was characterized by radioimmunoassay in two different systems: polyacrylamide gel electrophoresis and carboxymethyl cellulose chromatography. Both urines did not produce the same kinin: the kinin produced by human urine migrated like Lys-bradykinin, whereas the kinin produced by rat urine migrated like bradykinin. This gives evidence of differences in the specificity between kinin-forming enzymes in rat and human urines.