Albumin, transferrin, alpha1-acid glycoprotein, IgA, IgG, IgM, lysozyme and C3-complement factor have been immunologically determined in sputum and serum samples from 16 patients with chronic bronchitis. The sputa were effectively solubilized prior to the analysis. This is necessary for correct determination of the compositions of sputum. IgA (approx. 3 g/l) and lysozyme (approx. 1 g/l) were present in the highest concentrations. Lactoferrin was qualitatively shown to be present in all the sputa. The concentration of IgG, albumin and transferrin were much higher in the sera than in the sputa, their presence in sputum probably being a result of a passive" leakage" from serum. The ratios for IgA, IgM and lysozyme indicated that these macromolecules are locally synthesized in the respiratory tract. The concentrations of IgA and lysozyme were closely correlated, indicating that the biosynthesis of secretory IgA and bronchial lysozyme may be coupled or controlled by the same mechanism. Except for a weak correlation between the concentration of IgA in sputum and viscosity, no such correlations were obtained for the other proteins determined.