This invention provides methods and materials for treating cancer. The invention encompasses methods and materials for delivering programmed death-ligand 1 (PD-L1) binding compounds and/or compositions containing one or more monovalent or multivalent programmed death-ligand 1 (PD-L1) binding compounds which are administered to a mammal having cancer to treat the mammal. In some cases, a multivalent PD-L1 binding compound can include two or more programmed cell death protein 1 (PD-1) polypeptides (and/or fragments thereof having the ability to bind PD-L1). This invention also provides methods and materials for making multivalent PD-L1 binding compounds and methods and materials for making nucleic acid molecules that encode PD-L1 binding compounds.