The primary objective of the study is to evaluate the safety and tolerability of a range of single BIIB054 doses, administered as a single intravenous (IV) infusion, in healthy participants and participants with early Parkinson's disease (PD). Secondary objectives of the study are to assess the serum pharmacokinetics (PK) profile of BIIB054 after single-dose administration and to evaluate the immunogenicity of BIIB054 after single-dose administration.

Only healthy participants will be enrolled in Cohorts 1 through 6. Only participants with early PD will be enrolled in Cohort 7

Key Inclusion Criteria:

1. All women of childbearing potential and all men must practice effective contraception during the study and for 6 months after their last dose of study treatment.

2. Must have a body mass index from 19 to 32 kg/m2, inclusive.

3. Must be in good health as determined by the Investigator, based on medical history, physical examination, and 12-lead ECG.

   For cohort 7 only:

4. Diagnosis of idiopathic PD

Key Exclusion Criteria:

1. History of cardiovascular disease.
2. Subject smokes more than 5 cigarettes or the equivalent in tobacco daily.
3. Surgery within 3 months prior to Day -1 (other than minor cosmetic surgery and minor dental surgery, as determined by the Investigator).
4. History or positive test result for hepatitis C, hepatitis B, or human immunodeficiency virus (HIV).
5. Female subjects who are pregnant, currently breastfeeding, or attempting to conceive during the study.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.