The study will examine the safety profile of SGN-CD19B administered as a single agent. The main purpose of the study is to estimate the highest dose that does not cause unacceptable side effects of SGN-CD19B in patients with relapsed or refractory aggressive B-cell non-Hodgkin lymphoma (NHL) subtypes of diffuse large B-cell lymphoma (DLBCL) and Grade 3 follicular lymphoma (FL3). Additionally, the pharmacokinetic profile and antitumor activity of SGN-CD19B will be assessed.