Background: The anti–PD-L1 monoclonal antibody atezolizumab inhibits the interaction of PD-L1 with its receptors PD-1 and B7.1, thereby restoring T-cell immunity. In the Phase III OAK study, patients with previously treated advanced NSCLC had improved mOS in the atezolizumab arm (13.8 mo) vs the docetaxel arm (9.6 mo) (HR 0.73 [95%CI: 0.62, 0.87]; P= 0.0003), irrespective of PD-L1 expression or histology. A Phase III clinical trial of atezolizumab monotherapy for first-line, PD-L1–selected patients with NSCLC is underway; however, first-line atezolizumab monotherapy for NSCLC treatment in a biomarker-unselected population has not yet been investigated. Current assays to measure PD-L1 expression by IHC require tumor biopsies, which can be difficult to obtain in some patients. Novel blood-based biomarkers will be evaluated retrospectively in B-F1RST (Blood-First-Line Ready Screening Trial) in patients receiving atezolizumab monotherapy in first-line NSCLC. Methods: A Phase II, open-label, single-arm study, B-F1RST (NCT02848651), will evaluate the efficacy and safety of atezolizumab in PD-L1–unselected patients with first-line locally advanced or metastatic NSCLC. Eligibility criteria include stage IIIB-IVB NSCLC, ECOG PS 0-1, measurable disease per RECIST v1.1 and adequate hematologic and end-organ function. Exclusion criteria include the presence of EGFRmutations or ALKfusions, active CNS metastases and prior immunotherapy for NSCLC. Patients will receive atezolizumab 1200 mg IV q3w until disease progression or loss of clinical benefit. Prospective collection of blood samples is mandatory; collection of tissue biopsies is optional. The co-primary endpoints of the study are investigator-assessed ORR per RECIST v1.1 for the efficacy objective and PFS per RECIST v1.1 for evaluating blood-based predictive biomarkers for atezolizumab efficacy, including mutation status. Approximately 150 patients will be enrolled at 25 or more centers in the United States. Clinical trial information: NCT02848651