The aim of the study was to estimate colonic mucus physicochemical state in children with ulcerative colitis. Investigations were performed in 36 children, aged 10-17 years, allotted into three groups dependently on the severity of the disease. The severity of the disease was estimated basing on Truelove-Witts criteria. Mucus analysis was performed with polyacrylamide gel electrophoresis (SDS-PAGE), electron paramagnetic resonance (EPR) and spectrophotometric method. Mucus fraction dissolved in normal saline was subject of investigations. Protein content was determined in this fraction with Lowry's method. The obtained results demonstrated different mucus solubility dependent on the severity of the disease. Mucus protein concentration increased whereas mucus viscosity decreased with the severity of the disease. Colonic mucus physicochemical properties change dependently on the disease severity. Determination of soluble protein fraction in mucus may be a useful marker in the estimation of the disease severity.