• Approximately 45 patients suffering from mild to moderate ulcerative colitis will be examined for any side effects (safety and tolerability) that may occur when taking GLPG0974 or matching placebo (2:1 ratio) for 28 days.
• During the course of the study, patients will also be evaluated for improvement of disease activity by different efficacy measures, the amount of GLPG0974 present in the blood determined (PK) and the effects of GLPG0974 on selected biomarkers (PD) in the blood, colon biopsies and faeces will be explored.

Key Inclusion Criteria:

• Men or women between 18 to 75 years of age inclusive, on the day of signing the informed consent with a documented history of UC.
• Presence of mild-to-moderately active ulcerative colitis as evidenced by clinical signs and endoscopy.
• Medication: 5-ASA.
• Absence of infectious colitis.

Key Exclusion Criteria:

• History of sensitivity to any component of the study drug
• Any concurrent illness, condition, disability or clinically significant abnormality (including lab tests) that represents a safety risk , may affect the interpretation of data, or may prevent the subject to safely complete the assessments
• Positive serology for HIV 1 or 2 or hepatitis B or C, or any history of HIV or hepatitis.
• History of active infections requiring intravenous antibiotics within the past four weeks prior to randomization.
• History of bowel surgery, or presence or history of intestinal malignancy.
• Suspicion of Crohn's disease, indeterminate colitis, microscopic colitis, ischaemic colitis or radiation-induced colitis.
• History of lower GI bleeding disorder, other than UC.
• A history of significant psychological, neurologic, hepatic, renal, endocrine, cardiovascular, gastrointestinal (other than UC), pulmonary or metabolic disease.
• History of tuberculosis (TB) infection.
• Treatment with systemic corticosteroids within 1 week prior to randomization.
• Treatment with TNF-α inhibitors or other biologics within 2 months prior to randomization.
• Treatment with immunosuppressants, initiated or changed within 3 months prior to randomization .
• Current use of probiotic or prebiotic preparations
• Regular daily use of NSAIDs, within 7 days prior to randomization.
• Administration of any experimental therapy within 90 days or 5x the half-life.
• History of drug or alcohol abuse.
• Pregnant or lactating women.
• Medical, psychiatric, cognitive, or other conditions that compromise the subject's ability to complete the study.