Monomethyl auristatin E (MMAE) is a potent tubulin inhibitor that is used as the payload for four FDA-approved antibody drug conjugates (ADCs). Deconjugated MMAE readily diffuses into untargeted cells resulting in off-target toxicity. Here we report the development and evaluation of a humanized Fab fragment (ABC3315) that enhances the therapeutic selectivity of MMAE ADCs. ABC3315 increased the IC50 of MMAE against human cancer cell-lines by >500-fold with no impact on the cytotoxicity of MMAE ADCs, including polatuzumab vedotin and trastuzumab-vc-MMAE (TvcMMAE). Co-administration of ABC3315 did not reduce the efficacy of polatuzumab vedotin or TvcMMAE in xenograft tumor models. Co-administration of ABC3315 with 80 mg/kg TvcMMAE significantly (p<0.0001) increased the cumulative amount of MMAE that was excreted in urine 0-4-days after administration from 789.4 ±19.0 nanograms (TvcMMAE alone) to 2625±206.8 nanograms (for mice receiving TvcMMAE with co-administration of ABC3315). Mice receiving 80 mg/kg TvcMMAE and PBS exhibited a significant drop in white blood cell counts (p=0.025) and red blood cell counts (p=0.0083) in comparison to control mice. No significant differences, relative to control mice, were found for white blood cell counts (p=0.15) or for red blood cell counts (p=0.23) for mice treated with 80 mg/kg TvcMMAE and ABC3315. Co-administration of ABC3315 with 120 mg/kg polatuzumab vedotin significantly (p=0.045) decreased the percentage body weight loss at nadir for treated mice from 11.9 ± 7.0% to 4.1 ± 2.1%. Our results demonstrate that ABC3315, an anti-MMAE Fab fragment, decreases off-target toxicity while not decreasing anti-tumor efficacy, increasing the therapeutic window of MMAE ADCs.