The natural racemic Gossypol has two enantiomers, namely the (R)-(-)-Gossypol (AT-101) and (+)-Gossypol enantiomers. (R)-(-)-Gossypol (AT-101) and (+)-Gossypol binds to Bcl-2 or Bcl-xL with similar binding affinities, AT-101 is more potent than (+)-Gossypol in inhibition of cell growth and induction of apoptosis, possibly due to the influence of serum in the cell culture experiments. The racemic form and each of the enantiomers of Gossypol are tested against UM-SCC-6 and UM-SCC-14A in 6-day MTT assays. AT-101 exhibits greater growth inhibition relative to (±)-Gossypol than (+)-Gossypol in both cell lines tested (P<0.001). An intermediate growth inhibitory effect is observed with (±)-Gossypol but this effect is only observed at the higher dose of Gossypol (10 μM, P<0.0001). (R)-(-)-Gossypol (AT-101) binds to the BH3-binding groove of Bcl-xL and Bcl-2 proteins with fairly high affinity, has potent activity against head and neck squamous cell carcinomas (HNSCC) cell lines in vitro. Furthermore, it induces apoptosis with high efficiency in HNSCC tumor cells that express functional p53 and that also kills tumor cells with mutant p53 by a different mechanism. AT-101 doses required to inhibit the growth of human fibroblast cell lines by 50% were 2- to 10-fold higher than for HNSCC cell lines. To inhibit human oral keratinocyte growth by 50%, (R)-(-)-Gossypol (AT-101) concentrations are 2-to 3-fold higher than for HNSCC cell lines. (R)-(-)-Gossypol (AT-101) causes dose-dependent inhibition of cell growth in ten UM-SCC cell lines over a range from 0.5 to 10 μM in a 6-day MTT assay. The relative sensitivity of the cell lines vary from a very sensitive group with an IC ₅₀ of 2-5 μM and a less sensitive group with IC ₅₀ clusters around 10 μM. (R)-(-)-Gossypol (AT-101) is determined to bind to Bcl-2, Mcl-1 and Bcl-xL proteins with K _i values of 260±30 nM, 170±10 nM, and 480±40 nM, respectively.