We have studied the sputum/serum protein concentration ratios from 23 patients with bronchitis both in the stable clinical state and during acute chest infections. During the stable state there was a significant negative correlation (2 P less than 0.005) between the ratio and protein size. The ratios of IgG, IgA, C3, and alpha 1-antichymotrypsin were significantly displaced from this relation suggesting local production in the lung. IgM was found in all samples and alpha 2-macroglobulin in 55% of non-infected samples which may be the result of local production rather than transudation from serum, because of their larger size. During acute chest infections the albumin content of sputum rose from a mean sputum/serum ratio of 0.83 (SE +/- 0.08) X 10(-2) to 13.77 (SE +/- 3.21 X 10(-2) suggesting increased transudation from the blood. In the presence of increased transudation, local production of protein appears to be less significant.