Antibodies to rhGM-CSF generated in immunized patients are not neutralizing in activity. IgG purified from patients CR4723 and MT8463 preimmunization and after all six immunizations was diluted in media and cultured with 10⁴ TF-1 cells for 48 hours. At the end of 48 hours, cultures were pulsed for 8 hours with 1 μCi³H-thymidine and the incorporated radioactivity counted. The results are reported as the mean cpm of triplicate wells.