Study of XmAb942 in Healthy Participants and Participants With Ulcerative Colitis

Inclusion Criteria: Parts A and B * Age 18-55 * Must be in good health with no significant medical history * Clinical laboratory values within normal range * BMI 18-35 (inclusive) * Contraceptive use by men or women consistent with local regulations * Able and willing to provide written informed consent Part C * Age 18-75 * Must be in good health with no significant medical history * UC diagnosis ≥ 3 months prior to screening * Diagnosis of moderately to severely active UC as defined by a (MMS) ≥ 5, with a MES ≥ 2 and RBS ≥ 1 * Evidence of UC extending ≥ 15 cm from the anal verge, as determined by screening colonoscopy * Must have inadequate response to, loss of response to, or intolerance to at least 1 of the conventional or advanced therapies of UC * Able and willing to provide written informed consent Exclusion Criteria: Parts A and B * Any physical or psychological condition that prohibits study completion * History of suicidal behavior or suicidal ideation * Heavy use of nicotine containing products * HIV, hepatitis B and hepatitis C positive * Cardiac arrhythmia, or clinically significant abnormal ECG * Active use of prescription medications within 14 days of Day -1 * Active use of over-the-counter, or herbal medication within 7 days of Screening * Other investigational products within 30 days * Blood or plasma donation within 60 days * Pregnant or breastfeeding Part C * Any physical or psychological condition that prohibits study participation * Diagnosis of Crohn disease, indeterminate colitis, indeterminate colitis, microscopic colitis, ischemic colitis, infectious colitis, radiation colitis, and diverticular disease associated with colitis. * Positive screen for Clostridium difficile (C. Difficile) toxins * HIV, hepatitis B and hepatitis C positive * Cardiac arrhythmia, or clinically significant abnormal ECG * Pregnant or breastfeeding Other protocol defined inclusion/exclusion criteria apply.