In0.52Al0.48As/In0.39Ga0.61As0.77Sb0.23/In0.53Ga0.47As double heterojunction bipolar transistors (DHBTs) were irradiated with 5 MeV protons at fluences from 2x10(11) to 2x10(15) protons/cm(2). The radiation produced significant increases in generation-recombination leakage current in both emitter-base and base-collector junctions. The DHBTs irradiated with a dose of 2x10(11) cm(-2), which was equivalent to around 40 years of exposure in low Earth orbit, showed minimal changes in the junction ideality factor, generation-recombination leakage current, current gain, and output conductance. The InAlAs/InGaAsSb/InGaAs DHBTs appear to be well suited to space or nuclear industry applications.