The selective crystallization behavior of a series of commercial medium- and high-density polyethylene resins has been studied by means of an original modified procedure of the stepwise isothermal segregation technique using differential scanning calorimetry. The technique consists of a sequenced multiple-stage stepwise thermal treatment of the materials allowing separation of the macromolecules with respect to their length-to-branching content and distribution. It is assumed that such a separation process gives an image of the proportion of specific crystallizable species, which are in turn responsible for slow crack, growth resistance of the resins under study. A drift molecular parameter is calculated from a combination of the crystallization data at 119 degrees C and 114 degrees C. This molecular parameter is capable of revealing some divergences regarding the fabrication of a commercial polyethylene resin within a decade. Such divergences are well correlated with premature brittle failures under hydrostatic pressure testing of a series of correctly extruded pipes. Furthermore the drift molecular parameter allows a ranking of different medium- to high-density polyethylene resins-the lower this parameter the better the slow crack growth resistance of the materials.