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The authors assessed plasma prekallikrein (PK) after its conversion into the active enzyme kallikrein from the rate of breakdown of the specific chromogenic substrate NO-Pro-Pre-Arg-pNA. Activation of PK was achieved by the parallel contact method, using dextran sulphate (DS) and by direct activation by the Hageman's factor fragment (HFf). Contact activation assumes the presence of Hageman's factor (HF) and high molecular kininogen (HMW-K) in plasma. Both methods of activation and x = 0,451 +/- 0.146 U/ml in HFf activation). The mean PK value in neonates assessed by activation of DS was lower [mean = 0.105 +/- 0.068 U/ml] than the value assessed by activation of HFf [mean = 0.175 +/- 0.072 U/ml]. In adults the PK values varied at a significantly higher level, the results being higher when HFf was used as activator [mean = 0.346 +/- 0.087 U/ml in DS activation and mean = 0.451 +/- 0.146 U/ml in HFf activation]. The increase of mean values of PK assessed in umbilical blood and on the 3rd and 5th day after delivery was more marked in direct HFf activation. The correlation coefficient of both ways of assessment in neonates was r = 0.79, in the group of adults there was a close correlation r = 0.92. The lower correlation in neonates can be explained by the low level of co-factors, in particular HMW-K, necessary for complete activation of PK by the contact method. In some neonates the PK value assessed by DS expresses only the total activating plasma capacity and not the actual PK level.