Disclosed is a bispecific antibody comprising a first antigen combining site that binds to a CD200 protein and a second antigen combining site that binds to a CD20 protein. Also disclosed is the use of an anti-CD200 antibody, or a CD200-binding fragment thereof, in the preparation of a medicament for treating a human afflicted with a cancer that is resistant to therapy with an anti-CD20 therapeutic agent. Further disclosed is the use of an anti-CD200 antibody, or a CD200-binding fragment thereof, in the preparation of a medicament for treating a human afflicted with a cancer comprising cancer cells that express CDS.