Background A gamma irradiation dose of 15 kGy has been shown to adequately sterilise allograft bone, commonly used in femoral impaction bone grafting to treat bone loss at revision hip replacement, without significantly affecting its mechanical properties. The objective of this study was to evaluate whether use of 15 kGy irradiated bone affects the initial mechanical stability of the femoral stem prosthesis, as determined by micromotion in a comprehensive testing apparatus, in a clinically relevant time zero in vitro model of revision hip replacement. Methods Morselised ovine bone was nonirradiated (control), or irradiated at 15 kGy or 60 kGy. For each dose, six ovine femurs were implanted with a cemented polished taper stem following femoral impaction bone grafting. Using testing apparatus that reproduces stem loading, stems were cyclically loaded and triaxial micromotion of the stem relative to the bone was measured at the proximal and distal stem regions using non-contact laser transducers and linear variable differential transformers. Findings There were no significant differences in proximal or distal stem micromotion between groups for all directions (p ≤ 0.80), apart for significantly greater distal stem medial-lateral micromotion in the 60 kGy group compared to the 15 kGy group (P = 0.03), and near-significance in the anterior-posterior direction (P = 0.08, power = 0.85). Interpretation Using a clinically relevant model and loading apparatus, irradiation of bone at 15 kGy does not affect initial femoral stem stability following femoral impaction bone grafting.