Faust first reported the news through the research site Brief19.
Eli Lilly said it supports the decision by an independent monitoring board to "cautiously ensure the safety of the patients participating in this study," according to spokeswoman Kathryn Beiser.
Details regarding the safety concern are not yet known. A review from the independent data and safety monitoring board will be completed before further action is taken.
The ACTIV-3 trial was set up to test the Eli Lilly-developed monoclonal antibody in combination with remdesivir. Monoclonal antibodies are made to act as immune cells that scientists hope can fight the virus. A blood sample from one of the first U.S. patients who recovered from COVID-19 was used for the experimental antibody treatment.
Other companies, such as Regeneron Pharmaceuticals and AstraZenca pharmaceutical company, are also working on antibody treatments.
President Donald Trump received a monoclonal antibody treatment made by Regeneron along with remdesivir earlier this month.
The ACTIV-3 trial is one of several ongoing trials, which are part of the National Institute of Health's "Activ" program set up to accelerate development of COVID-19 vaccines and treatments.
The trial is supported by Operation Warp Speed, a Trump administration effort to make and distribute vaccines to fight COVID-19.
Worldwide, COVID-19 has infected over 37.9 million and killed over 1 million people.