The variability of sputum sol phase proteins was studied in 27 patients with stable chronic obstructive bronchitis. Within individual patients (n 1/2 11), variability over 5 consecutive days was similar for each of 4 proteins studied: average coefficient of variation (CV) for albumin was 39.8% for alpha 1-antitrypsin (alpha 1AT), 32.1%, for alpha 1-antichymotrypsin (alpha 1ACh), 26.6%, and for immunoglobulin A (IgA), 35.1%. The calculation of sputum-to-serum ratios did not affect this variability. Between patients (all patients studied on a single day) the variability of sputum sol phase proteins was greater (CV: albumin 1/2 84.5%, alpha 1AT 1/2 90.1%, alpha 1ACh 1/2 58.3%, and IgA 1/2 71.6%) and was also unaffected by the use of sputum-to-serum concentration ratios. "Standardization" for albumin reduced the average within-patient variability for sputum alpha 1AT (CV from 32.1 to 19.7%) but it had no effect on alpha 1ACh and IgA. However, although between-patient CV for sputum alpha 1AT was also reduced (from 90.1 to 53.4%, 2 p less than 0.01) it significantly increased for alpha 1ACh (from 58.3 to 86.0%, 2 p less than 0.05) and IgA (71.6 to 170.1%, 2 p less than 0.001), suggesting that such standardization may be inappropriate for these proteins.