<h3>Introduction</h3> Seminal studies of peripheral blood versus bone marrow grafts have elucidated the crucial role of CD3+ T-cells in mediating engraftment, immune reconstitution, disease control, and graft versus host disease (GVHD). An optimal dose-response relationship between the T-cell dose in the graft and outcomes is yet to be defined. Therefore, we performed a phase II clinical trial to evaluate whether T-cell dose standardization would adequately support engraftment and immune reconstitution while mitigating the risk of GVHD. <h3>Methods</h3> We conducted a clinical trial of dose standardization of T-cells between December 2014 and May 2018. Eligible adult patients (pts) with hematologic malignancies undergoing peripheral blood matched sibling allogeneic stem cell transplant were enrolled. Standardized T-cell dose was targeted 15-20 × 10e7. Tacrolimus and methotrexate were utilized for GVHD prophylaxis. The primary endpoint was to determine the incidence of grade II-IV acute GVHD. Secondary endpoints included overall survival (OS), progression-free survival (PFS), non-relapse mortality (NRM), and time to engraftment. Chronic GVHD was captured using conventional criteria. <h3>Results</h3> 17 pts were included in the study. Peripheral blood stem cell (PBSCT) graft from matched sibling donors was used in all pts. The mean age was 52 years (20-68 years). 53% pts were female and 82% were Caucasian. The median T-cell dose was 19.1 × 10e7 (range 9.1-21.8 × 10e7). Indications for PBSCT included AML/MDS (70.6%), ALL (17.6%), NHL and MPN (11.8 %). 35% pts received myeloablative and 65% received reduced-intensity conditioning. 59% had age-adjusted HCT-CI scores ≥3. 16 pts received tacrolimus and methotrexate for GVHD prophylaxis, with 1 patient receiving tacrolimus and mycophenolate. All pts successfully engrafted. The median time to engraftment was 15 days for neutrophils (range 10-20), and 12 days for platelets (range 9-22). The cumulative incidence of grade II-IV and grade III-IV acute GVHD at 1 year was 27% (95%-CI: 7-52) and 22.5 (95%-CI: 4.7-48.4) (no grade IV) and that of chronic GVHD was 47% (22-69) (N=8, all extensive by conventional criteria). 18% pts reactivated CMV (median - 40 days, range 33-50). 47% pts were alive in remission after a median follow-up of 18.1 months (3.3-52.2). The cumulative incidence of relapse at 1 year was 23.5% (95% CI: 7-46) (Figure 1). The 1 year PFS was 53% (95% CI: 28-73), and the OS was 65% (95% CI: 37-82). <h3>Conclusion</h3> Administration of a lower standardized T-cell dose is feasible, and when used in combination with standard immune suppression did not negatively affect outcomes. This study paves the way for a larger trial to evaluate the optimal T-cell dose and immune suppressive therapy to strike a favorable GVHD/GVL balance. Such a strategy could be used as a platform for additional interventions aimed towards improving the outcome of PBSCT for hematologic malignancies.