Study Title: Maxfield_T001 PGT121 Related studies: Barouch_628.01 and Barouch_628.02 This study is a phase 1 first-in-human randomized placebo-controlled clinical trial to determine the safety, pharmacokinetics and antiviral activity of PGT121 monoclonal antibody (mAb) in HIV-uninfected and HIV infected adults. A single intravenous (IV) infusion of PGT121 mAb at 3, 10, or 30 mg/kg in HIV-uninfected adults and HIV-infected adults will be given. The pharmacokinetics (PK) of the mAb after administration to the study participant will be determined by measuring PGT121 mAb in serum by a customized luminex assay and by a neutralizing antibody assay to determine if the antibody remains functional. It is our plan to use the CAVD CAVIMC CFS for these assays. The time points for these PK assays will be as follows: at screening, on the day 0 infusion visit (serum collected at 7 timepoints as described below), and at days 1,2,3,7,14,21,28,42,56,70,84,112,140, and 168 (end of study). During the day 0 infusion visit, the PK draws for serum will be done immediately before IP administration, at the end of the administration of IP, and at 30 minutes and 3 hours post end of the IP administration. Additional PK draws on day 0 are done 6, 9, and 12 hours after the start of the IP administration. This study includes the measurement of PK via binding antibody assays.