Clinical experience supports that tooth bleaching is effective without prophylaxis.  Objective: To quantitatively verify this observation under controlled conditions, we examined the proportional efficacy of Crest Whitestrips (CWS) bleaching on stained and prophylaxis cleaned (pre stained) enamel surfaces in vitro.  Methods:  Human enamel crowns were sectioned and mounted in methacrylate blocks.  Enamel was polished with prophylaxis paste and exposed to laboratory PCR staining.  Following staining surface color was determined quantitatively (Fuji Camera) as CIELAB L*, a*, b*.     ˝ of each stained surface was cleaned of extrinsic stains using prophylaxis paste and surface color of both sides were re-measured.  Teeth were then cycled for 14 days with continuous immersions in pooled human saliva at 37°C interrupted by bid ˝ hour CWS applications.  Controls  were brushed 2x/day with a slurry of non-peroxide dentifrice (Crest Dual Action Whitening - CDAW).  Color assessments on stained and prophied sides were made at days 1, 3 and 14. Results:   L* color response was the most reproducible for whitening assessments.  L* values were compared via Students t.  Baseline stained, day 14 stained and baseline prophy side L*:  CDAW 34.9±2.5; 69.6±2.2; 65.6±2.8; CWS 32.6±1.5; 74.6±3.3; 65.2±1.5.  There were nsd in prophied vs stain portions at day 14 (within treatments), though Day 1, 3, 14 L* was lighter for CWS treatment vs. CDAW paste treatment (e.g. at day 14 CWS  L*  stained 74.6±3.3 nsd vs. prophied 73.9±3.8  sig. > CDAW L* stained 69.6±2.2 nsd vs. prophied 70.3±2.9).  Conclusions:  The presence of extrinsic staining had no impact on final whitening results for CWS or CDAW dentifrice.  The dentifrice removed extrinsic stain only while CWS produced complete stain removal and intrinsic whitening as well.