Histone deacetylase inhibitors (HDACi) suppress cancer cell growth, inflammation, and bone resorption. The aim of this study was to determine the effect of inhibitors of different HDAC classes on human osteoclast activity in vitro. Human osteoclasts generated from blood mononuclear cells stimulated with receptor activator of nuclear factor kappa B (RANK) ligand were treated with a novel compound targeting classes I and II HDACs (1179.4b), MS-275 (targets class I HDACs), 2664.12 (targets class II HDACs), or suberoylanilide hydroxamic acid (SAHA; targets classes I and II HDACs). Osteoclast differentiation was assessed by expression of tartrate resistant acid phosphatase and resorption of dentine. Expression of mRNA encoding for osteoclast genes including RANK, calcitonin receptor (CTR), c-Fos, tumur necrosis factor (TNF) receptor associated factor (TRAF)6, nuclear factor of activated T cells (NFATc1), interferon-b, TNF-like weak inducer of apoptosis (TWEAK), and osteoclast-associated receptor (OSCAR) were assessed. Expression of HDACs 1-10 during osteoclast development was also assessed. 1179.4b significantly reduced osteoclast activity (IC 50 < 0.16 nM). MS-275 (IC 50 54.4 nM) and 2664.12 (IC 50 > 100 nM) were markedly less effective. A combination of MS-275 and 2664.12 inhibited osteoclast activity similar to 1179.4b (IC 50 0.35 nM). SAHA was shown to suppress osteoclast activity (IC 50 12 nM). 1179.4b significantly ( P < 0.05) reduced NFATc1, CTR, and OSCAR expression during the later stages of osteoclast development. Class I HDAC 8 and Class II HDAC5 were both elevated ( P < 0.05) during osteoclast development. Results suggest that inhibition of both classes I and II HDACs may be required to suppress human osteoclastic bone resorption in vitro.