Inhibition of osteoclast formation is an early event. Nonadherent cells from HBM and MMBM were cultured with 10 ng/mL CSF-M and 50 ng/mL RANKL for 21 days and DMSO (control), CC-4047, or thalidomide (Thal) was added twice a week to maintain a concentration of 0.1% and 100 μM, respectively. Drugs were added to (A) HBM and (B) MMBM for either the last week, the last 2 weeks, or for 3 weeks. Drugs were also added to (C) HBM or (D) MMBM for either the first 1 week, the first 2 weeks, or for 3 weeks. Cells were fixed and stained with 23c6 antibody to detect the multinucleated mature OCLs. Data shown are the mean plus or minus SD of multinucleated cells per well of at least 8 wells. *Significant difference from control (P < .05). All experiments were performed independently 3 times.