The average activity levels of acid alpha-glucosidase are comparable in liver supernatant fluids for 15 cystic fibrosis patients and 12 controls (401 +/- 131 and 347 +/- 109 nmol/h/mg protein, respectively) and no significant differences were found for the cystic fibrosis and control liver acid alpha-glucosidases in their (a) apparent Km values for the 4-methylumbelliferyl substrate (1.1 mmol/l), (b) pH optima (4.2) and thermostability curves and (c) isoelectric profiles (one form with an isoelectric point of 4.5 +/- 0.2). In contrast, average neutral alpha-glucosidase activity levels were significantly increased (p less than 0.0002) in sera from 21 cystic fibrosis patients compared to 15 controls (10.7 and 2.7 nmol/h/ml). This increased activity is not due to (a) different stability upon storage at --20 degrees C, (b) the presence of activators in cystic fibrosis sera or inhibitors in normal sera (as determined by mixing studies), (c) altered Km values or (d) altered pH optima curves. Cystic fibrosis serum neutral alpha-glucosidase appears to be more thermostable and has a consistently altered isoelectric profile (greater percentage of activity above pI 4.8) when compared to the normal serum enzyme. This altered isoelectric composition may reflect changes in neutral alpha-glucosidase which contribute to its increased activity in cystic fibrosis sera.