The main purpose of this study is to evaluate the safety of the study drug known as mirikizumab. The study will investigate how the body processes the study drug and how the drug affects the body. The study will last about 3 months for each participant.

Inclusion Criteria (Psoriasis participants):

• Chronic plaque psoriasis based on an investigator confirmed diagnosis of chronic psoriasis vulgaris for at least 6 months prior to baseline
• Plaque psoriasis involving >2% body surface area (BSA) in affected skin other than the face and scalp at screening and baseline
• Are willing and able to washout topicals for at least 14 days before baseline on the 2 target lesions

Inclusion Criteria (Healthy participants):

• Are overtly healthy males or females, as determined by medical history and physical examination
• Are women not of childbearing potential
• Are between the ages of 18 and 65 years, inclusive, at the time of screening
• Have a body mass index between 18.5 and 32.0 kilogram per square meter (kg/m2), inclusive, and a minimum body weight of 55 kg
• Have clinical laboratory test results within normal reference range for the central laboratory, or results with acceptable deviations that are judged to be not clinically significant by the investigator
• Are reliable and willing to make themselves available for the duration of the study and are willing to follow study procedures
• Have given written informed consent approved by Lilly and the ERB governing the site

Exclusion Criteria (Psoriasis and healthy participants):

• Have received treatment with biologic therapies for psoriasis (such as monoclonal antibodies, including marketed or investigational biologic therapy). Prior or current use of biologics for indications other than psoriasis may be allowed with sponsor approval
• Within 28 days prior to baseline: have received systemic nonbiologic psoriasis therapy
• Within 14 days prior to baseline: have received topical psoriasis treatment
• Have presence of significant uncontrolled cerebro-cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrine, hematologic, neurologic or neuropsychiatric disorders or abnormal laboratory values at screening that, in the opinion of the investigator, pose an unacceptable risk to the subject if participating in the study or of interfering with the interpretation of data
• Have had clinically significant symptomatic herpes zoster within 3 months of screening
• Show evidence of active or latent tuberculosis (TB)
• Have received live vaccine(s) (included attenuated live vaccines) within 1 month of screening or intend to during the study
• Have significant allergies to humanized monoclonal antibodies or any components of the mirikizumab product formulation or have a history of significant atopy
• Have had lymphoma, leukemia, or any malignancy within the past 5 years, except for basal cell or squamous epithelial carcinomas of the skin that have been resected with no evidence of metastatic disease for 3 years and cervical carcinoma in situ, with no evidence of recurrence within 5 years prior to baseline

Safety Population: all randomized participants who received any amount of study drug.