Pulmonary alveolar proteinosis (PAP) is characterized by the accumulation of lipoproteinaceous material within the lung alveoli. Recent studies indicate that PAP is an autoimmune disease characterized by a neutralizing anti-granulocyte macrophage colony stimulating factor (GM-CSF) antibody. At present the only definitive diagnostic test for PAP is open lung biopsy. We have previously published that anti-GM-CSF is diagnostic for PAP and correlates with disease pathogenesis using a traditional serial anti-GM-CSF antibody titer format (T. L. Bonfield, M. S. Kavuru, and M. J. Thomassen, Clin. Immunol. 105:342-350, 2002). Titer analysis is a semiquantitative method, and often subtle changes in antibody titer are not detectable. In this report we present data to support anti-GM-CSF detection by a quantitative highly sensitive multiplexed particle-based assay which has the potential to be a clinical diagnostic test.