HDPE was gamma-irradiated at room temperature. The resistance to slow crack growth (SCG) was measured in single edge notched tensile specimens under constant load as a function of the dose. The resistance to SCG initially decreased to a minimum value at a dose between 0.05 and 0.10 Mrd. The minimum value was 45% less than for the undosed state. For doses greater than 0.10 Mrd, the resistance to SCG increased up to a dose of 50 Mrd, where its value had increased by a factor of 10(2). The gel point occurred at 1-3 Mrd. MI and the crack opening displacement exhibited maximum values at a dose of 0.1 Mrd. The behaviors of SCG, MI and crack opening displacement were consistent with the explanation that chain scission dominated for doses less than 0.1 Mrd, and cross-linking dominated at the higher doses. For doses beyond 50 Mrd, the resin became so brittle that it cracked during the loading of the specimen. Beyond the gel point the density increased from 0.9694 to 0.9716 g/cm3 at a dose of 160 Mrd.