Background: Patients (pts) with R/R CLL who discontinue treatment with the BTKi ibrutinib due to progression have a particularly dismal prognosis. A phase I study showed that the Fc-enhanced, humanized, CD19 antibody MOR208 was well tolerated with encouraging single-agent activity in pts with R/R CLL/SLL. In preclinical models, MOR208 showed synergy with idelalisib (an inhibitor of PI3K delta) and venetoclax (an inhibitor of BCL-2), both approved for the treatment of CLL. Methods: This two-cohort, phase II study will investigate MOR208 combined with idelalisib (cohort A) or venetoclax (cohort B) in pts with R/R CLL or R/R SLL and includes a safety run-in phase for each cohort, to be evaluated by an Independent Data Monitoring Committee. Key inclusion criteria: aged ≥18 years, R/R CLL/SLL while receiving a BTKi therapy or intolerance of such therapy, BTKi administered as a single-agent or in combination for at least 1 month as the most recent prior anticancer therapy, ECOG performance status of 0–2, and adequate organ function. Key exclusion criteria: transformed CLL/SLL or Richter’s syndrome, BTKi treatment within 5 days prior to study drug dosing, prior treatment with a CD19-targeted therapy, a PI3K inhibitor (cohort A) or a BCL-2 inhibitor (cohort B). Pts will be treated for a maximum of 24 (28-day) cycles or until disease progression. Treatment will be MOR208 12 mg/kg IV (weekly for the first 3 months, every second week for the next 3 months, and monthly thereafter) in combination with oral idelalisib 150 mg twice-daily or venetoclax administered on a weekly ramp-up dosing schedule to the recommended daily dose of 400 mg. Primary endpoint: overall response rate based on independent review; secondary and exploratory endpoints include: progression-free and overall survival, duration of response, safety, pharmacokinetics, MOR208 immunogenicity, quality of life and minimal residual disease negativity. 120 pts per cohort are planned. Clinical trial information: 2015-002915-14.