The concentrations of "secretory piece" and total IgA, and the amount of IgA present as the 11S and 7S components, were studied in sputum. The results showed that all were present in greater quantities in infected than non-infected samples. The secretory piece measurements were correlated with measurements of the 11S IgA component in the infected samples, suggesting that all of the secretory piece was attached to the 11S IgA as secretory IgA. However, in the non-infected samples, secretory piece measurements were greater than the 11S IgA measurements, suggesting the presence of free secretory piece. The ratio of sputum to serum concentration of 7S IgA in both groups of samples was several times higher than the respective albumin ratios (average values for infected samples = 3.08 x albumin; average values for non-infected samples = 5.52 x albumin). This suggests that most of the 7S IgA present in sputum is also locally produced within the lung.