The purpose of this study was to characterize the spatial distribution, number and size of CD34+ cells in fetal bone marrow. Thin sections of normal fetal bone marrow from lumbar vertebrae were stained using CD34 antibody QBend/10. Sections were used under light microscopy with various eyepiece graticules to make measurements of CD34+ cells in situ. Results showed that at mid- and late gestation, approximately 2% and 0·5% of fetal bone marrow cells were CD34+ respectively. The mean distance of CD34+ cells from the nearest trabecular bone surface was 61 ± 4 and 46 ± 4 μm, respectively, for mid- and late gestation. The mean distance to the nearest neighbour was 46 ± 5 and 105 ± 15 μm, and the mean distance to the nearest blood vessel was 13 ± 1 and 17 ± 2 μm respectively. The concentration of CD34+ cells in the peripheral region was 6·5 times greater than that at the centre of the sections. Overall, the percentage number of CD34+ cells decreased with gestational age. The cellular and nuclear diameters of CD34+ cells remained unchanged throughout mid- and late gestation at 5·4 ± 0·1 and 3·8 ± 0·1 μm respectively. This information will be used to calculate the natural background alpha-radiation dose to haemopoietic stem cells.