Human and rabbit kidney and urine contain an inactive form of kallikrein. Studies on the mRNA sequence suggested that the active form of the enzyme and the propeptide are linked by a peptide bond between a basic and hydrophobic amino acid. We studied the activation of prokallikrein by serine proteases and a neutral metalloproteinase, thermolysin, because serine proteases cleave the peptide chain after a basic amino acid and thermolysin before a hydrophobic amino acid. The activity of kallikrein was measured by RIA and with a fluorogenic peptide substrate. Trypsin was used as a standard reference activator. We found that human plasmin and plasminogen, activated by urokinase, activate prokallikrein. Pronase coupled to Sepharose also enhanced the activity of the renal kallikrein zymogen. On a molar basis, thermolysin was a more effective activator of prokallikrein than trypsin. The activation by thermolysin was blocked by the inhibitor phosphoramidon, but not by DFP or SBTI. These experiments indicate that, in addition to serine proteases, neutral metalloproteases of tissues may activate prokallikrein.