Methionyl-lysyl-bradykinin, a new kinin from ox blood.

1. The pseudoglobulin fraction from ox serum was maintained at acid pH for several days and was then incubated at pH7.5 and 37 degrees . At least two polypeptides capable of stimulating smooth muscle were produced and attempts were made to obtain these in the pure state. 2. Only one substance could be purified completely and this was found to be a new plasma kinin, methionyl-lysyl-bradykinin. 3. The biological properties of this new kinin were compared with those of bradykinin and with a sample of synthetic methionyl-lysyl-bradykinin. 4. The possible significance of these results in relation to plasma-kinin formation by kallikrein and other enzymes is discussed.