Background and Objectives Number of total nucleated cells is an important prognostic factor of unrelated bone marrow transplantation. The objective of our study was to further optimize donor selection by finding donor characteristics which predict the total nucleated cell concentration in unrelated bone marrow products. Material and Methods Data were collected retrospectively from the pre-donation medical records. The outcome variable was concentration of total nucleated cells in the final products. We investigated several candidate predictors, including demographic variables and peripheral blood counts. A backward selection procedure was performed, using multivariable multilevel analysis. Results A total of 471 donations were included. Significant predictors were increase of age (ratio per 5 years 0·971), cytomegalovirus (ratio 0·947 if positive), smoking (ratio 1·079 if yes), higher haemoglobin (ratio 1·052 per mmol/l), higher mono-nuclear cells (ratio 1·169 when mono-nuclear cells >2·69 compared to mono-nuclear cells <1·89 × 10E9/l), increasing number of whole blood donations (ratio 1·115 when blood donations >4 compared to blood donation <1), and larger body surface area (ratio 1·179 when body surface area >2·07 compared to body surface area <1·82 m2). The model was adjusted for collection volume. Conclusion Presence of all favourable factors was associated with a twofold higher concentration of total nucleated cells in the bone marrow harvest.