The CD4-binding site antibody VH3810109 (N6LS) demonstrated broad and potent neutralization activity and robust antiviral activity in people with HIV-1. We report safety, tolerability, pharmacokinetics, and anti-drug antibody (ADA) incidence for subcutaneous (SC) and intravenous (IV) N6LS. Safety and pharmacokinetics of a single dose of VH3810109 (also known as GSK3810109), administered either subcutaneously (SC) with rHuPH20 or intravenously (IV) [SPAN]) was an open-label, three-part, phase 1 study evaluating single-dose N6LS in adults without HIV (part 1, 20 mg/kg SC + recombinant human hyaluronidase PH20 [rHuPH20] 2,000 U/mL; part 2, 60 mg/kg IV; part 3, 3,000 mg SC + rHuPH20 2,000 U/mL). Over 24 weeks, adverse events (AEs), injection site reactions (ISRs), pharmacokinetics, and ADAs were monitored. Twenty-four participants (8/part) were enrolled and received a single N6LS dose. Overall AE incidence between SC doses was similar and higher compared with IV, driven by ISRs. No ISRs were reported for IV N6LS; for SC, 15/16 participants reported ISRs, and 17/32 events were grade 3 (all injection site erythema). All ISRs resolved without sequelae or treatment. All participants rated local reactions and pain as acceptable. No serious AEs or deaths occurred. Pharmacokinetics were as expected for a broadly neutralizing antibody; median terminal half-life ranged from 43 to 47 days. No ADAs were observed after IV N6LS (0/8); 5/16 participants had treatment-emergent ADAs after SC N6LS; however, a clear impact on pharmacokinetics was observed in only one participant. N6LS administered IV or SC + rHuPH20 had a favorable safety profile and was well tolerated. Results support the ongoing development of N6LS 3,000 mg SC + rHuPH20 and 60 mg/kg IV into phase 2b.CLINICAL TRIALSRegistered at ClinicalTrials.gov (NCT05291520).