Precursors of bone-resorbing cells reside in the proliferative monocyte (PM) population. Sorted non-proliferative (NP) and PM populations (Figure 1) were cultured on bovine bone (3 × 104 cells per slice) for 21 days in the presence of macrophage colony-stimulating factor (8,000 U/mL) and receptor activator of nuclear factor-kappa-B ligand (50 ng/mL). (a) The bone slices were stained with haematoxylin (magnification × 200). Arrows indicate pits on the bone surface. (b) Resorption pit area measured for four donors (see 'Higher bone resorption in the proliferative monocyte population following culture in M-CSF and RANKL' section). Values are means of percentage of resorbed bone ± standard error. For each donor, the mean values for the PM group are significantly greater than those for the correspondingly treated NP cells (P < 0.05).