This study will assess the safety, pharmacokinetic (PK), pharmacodynamic (PD) and the therapeutic potential of GSK2857916 in subjects with multiple myeloma (MM) and other hematologic malignancies that express B cell maturation antigen (BCMA). The hypothesis is that GSK2857916 can be safely administered to subjects with BCMA positive malignancies at doses where target engagement can be demonstrated. This study will determine if adequate target engagement of BCMA receptors translates into clinical benefit for subjects with MM and other BCMA positive hematologic malignancies. The study will consists of two parts: a Part 1 dose escalation phase and a Part 2 expansion phase for safety, and clinical activity testing. The study will enroll a total of approximately 80 subjects with relapsed/refractory MM or other BCMA-expressing hematologic malignancies.