Human polymorphonuclear leukocytes (PMN) have been reported to completely lack of DNA-dependent protein kinase (DNA-PK) which is composed of Ku protein and the catalytic subunit DNA-PKcs, needed for nonhomologous end-joining (NHEJ) of DNA double-strand breaks. Promyelocytic HL-60 cells express a variant form of Ku resulting in enhanced radiation sensitivity. This raises the question if low efficiency of NHEJ, instrumental for the cellular repair of oxidative damage, is a normal characteristic of myeloid differentiation. Here we confirmed the complete lack of DNA-PK in PMN protein extracts, and the expression of the truncated Ku86 variant form in HL-60. However, this degradation of DNA-PK was shown to be due to a DNA-PK-degrading protease in PMN and HL-60. In addition, by using a protease-resistant whole cell assay, both Ku86 and DNA-PKcs could be demonstrated in PMN, suggesting the previously reported absence in PMN of DNA-PK to be an artefact. The levels of Ku86 and DNA-PKcs were much reduced in PMN, as compared with that of the lymphocytes, whereas HL-60 displayed a markedly elevated DNA-PK concentration. In conclusion, our findings provide evidence of reduced, not depleted expression of DNA-PK during the mature stages of myeloid differentiation.