The sera of 19 silica-dust-exposed subjects and of an equal number of age-, sex- and socioeconomic-strata-matched controls were analysed for antinuclear factor, rheumatoid factor, C-reactive protein, immunoglobulins G, M, A, and complement C3 and C4. Circulating immune complexes were also precipitated in all subjects and their immunoglobulin and complement C3 and C4 were estimated. Silica-exposed subjects were divided into two groups depending upon the radiological findings and it is suggested that IgA plays an important role in the immunopathogenesis of the disease and that lung changes could be due to the immune-complex-mediated mechanisms utilizing an alternative complement pathway.