The disulphide/thiol (SS/SH) content and rheological properties of highly purified small-intestinal mucins from normal (N) subjects and patients with cystic fibrosis (CF) were investigated.(1) An assay was developed to measure free SH groups (before reduction) and total SH content (after reduction) using 4, 4'-dipyridyl disulphide. SS bonds were calculated by difference. Experimental values for the SS and SH contents of well-characterized proteins obtained with the assay showed good agreement with expected values.(2) The SS and free SH contents of nine N and six CF mucins were variable: 44.4+/-5.4 nmol of SS and 4.3+/-1.1 nmol of free SH per mg of N mucin and 31.7+/-7.6 nmol of SS and 7.5+/-3.7 nmol of SH per mg of CF mucin. N and CF mucins were not statistically different. In most mucins, approximately 90% of the SH groups were involved in SS bonds.(3) Gels were reconstituted from the same N and CF mucins at concentrations between 8 and 25 mg/ml and their rheological properties were assessed by using a magnetic microrheometer.(4) Once formed, mucin gels were stable and maintained the same mechanical properties over a long period of time (3-14 days).(5) The rheological profiles of both N and CF samples did not vary with the concentration of mucin present and were characteristic of weak, visco-elastic gels.(6) Although variations were seen in the visco-elastic properties of individual mucins, no systematic differences were detected between N and CF preparations.(7) There was no apparent correlation between the rheological properties of a mucin and its SS/SH content.