Multiple isotypes of anti-GM-CSF antibodies are present in a subset of patients with AML and CML and are not induced by vaccine. (a) GM-CSF was resolved in 15% SDS-PAGE into three species with molecular weights of 19.5, 16.8 and 15.5 kDa. Western blotting with sera of four representative vaccinated patients (P15, P16, P18 and P19) and consequent probing with antibodies specific to human IgG, IgA and IgM revealed corresponding subclasses of anti-GM-CSF Igs. Antibody MAb 215 (anti-human GM-CSF) and mouse IgG1 were used as positive (P) and negative (N) controls, respectively. (b) Anti-GM-CSF IgA and (c) anti-GM-CSF IgM were present in patients' sera before (−) and after (+) vaccination. ELISA, enzyme-linked immunosorbent assay; GM-CSF, granulocyte-macrophage colony-stimulating factor; Igs, immunoglobulins; MAb, monoclonal antibody; SDS-PAGE, SDS-polyacrylamide gel electrophoresis.