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.