A group of 102 obligate heterozygotes for the cystic fibrosis gene were examined for genetic type of pancreatic isoamylases as well as enzyme activity of pancreatic isoamylases, salivary isoamylases and total amylase in serum. The analysis was performed using electrophoretic separation for detection of the various types of isoamylases, and their relative activity was determined by means of densitometry. The activities of salivary isoamylases and total amylase in serum showed no significant differences between the cystic fibrosis heterozygotes and a control group. The frequency of the variant gene for pancreatic isoamylase was also the same as in the controls. The mean value for pancreatic isoamylase was slightly higher in the cystic fibrosis heterozygotes than in the control group (0.05>P>0.02).There was no correlation between pancreatic isoamylase values and the three types of cystic fibrosis on the basis of metachromatic staining of cultured fibroblasts.