Solid cylindrical specimens (½- × ½-in.) of the monocarbides of Ti, Zr, Ta, Nb, and W, made by 1) hot pressing, 2) slip casting and sintering, and 3) explosion-pressing and sintering, were irradiated at 300 to 700°C. Fast-neutron (> 1 MeV) exposures ranged from 0.8 to 5.4 × 1021 n/cm2 in a fast-neutron flux profile which ranged from 0.6 to 2.6 × 1014 n/(cm2 sec). The order of decreasing fracture of specimens made by 1) and 2) was Ta, Zr, Nb, Ti, and W. Specimens made by 3) not only fractured at lower neutron doses than those made by 1) and 2), but there was also less difference in gross damage among the five carbides. Tungsten carbide expanded in volume by ∼0.6% and the other carbides by 2 to 3% upon exposure to fast doses of 1 to 2 × 1021 n/cm2. Higher doses produced either a decrease in volume toward the initial volume or no further change. Volume changes represented crystal volume changes since there was no grain boundary separation. Less than 50% of the crystal expansion was accounted for by increases in lattice parameters. The major cause of damage to carbides is postulated to result from point defects produced by fast neutrons. It is suggested that most of the initial volume expansion is caused by the formation of defect agglomerates too large to affect measured values of the lattice parameters. Slow neutrons of the irradiation spectrum may have contributed to premature fracturing of explosion-pressed specimens through absorptions by added Co and Ni binder at the grain boundaries.