Osteoclasts have been defined as calcitonin (CT) and vitronectin (VN) receptor (R) positive, and CD14‐, CD11b‐ and CD11c‐negative cells which resorb bone. The aim of this study was to identify the phenotype of the osteoclast precursor. Osteoclasts were generated by co‐culturing peripheral blood mononuclear cells (PBMNCS) with the rat osteoblastic UMR 106 cell line. On days 2–4 at least 80% of CTR‐positive cells co‐expressed CD14, CD11b and CD11c (monocyte markers), but by day 14 < 3.3% expressed these markers. Selection of CD14‐positive monocytes from PBMNCS enhanced osteoclastic bone resorption 2–4‐fold compared to unfractionated PBMNCS.

This study demonstrates that osteoclasts derive largely from CD14‐positive monocytes.