Abstract: Methods of therapy for treating a subject for chronic lymphocytic leukemia are provided. The methods comprise administering a therapeutically effective amount of an antagonist anti-CN40 antibody or antigen-binding fragment thereof to a patient in need thereof. The antagonist anti-CD40 antibody or antigen-binding fragment thereof is free of significant agonist activity, but exhibits antagonist activity when the antibody binds a CD40 antigen on a human CD40-expressing cell. Antagonist activity of the anti-CD40 antibody or antigen-binding fragment thereof beneficially inhibits proliferation and/or differentiation of human CD40-expressing chronic lymphocytic leukemia cells.