Several common mucolytic agents were screened for their ability to solubilize cervical mucus without incurring irreversible losses in mucus viscoelasticity. Sodium thiocyanate was selected on the basis of its effectiveness and handling convenience, and solubilized mucus was fractionated by gel filtration on Sepharose columns. The excluded mucin fraction was characterized by a high neutral hexose to protein ratio and contained all of the elasticity of thiocyanate-treated whole mucus. The concentration of mucin in individual mucus samples collected throughout the ovulatory menstrual cycle was determined from column elution profiles. An increased relative amount of mucin was associated with samples collected during the ovulatory phase of the cycle, and a strong correlation was obtained when the percentage of mucin in individual samples was plotted against their respective storage moduli. These results indicate that the cyclic fluctuations in mucus viscoelasticity seen previously at constant concentrations of nondialyzable solids can be attributed to variations in mucin concentration. Evidence was also obtained, however, for compositional differences in mucus derived from different patients.