Kent Brantly and another American missionary worker, Nancy Writebol, "are in stable but grave condition," said a statement from Samaritan's Purse, the US group for which Brantly worked.
"Dr. Brantly took a slight turn for the worse overnight," it said.
The group did not elaborate further on Brantly's condition.
It said that when an experimental serum arrived in the capital, Monrovia, on Wednesday, there was only enough for one person and Brantly had asked for it to be given to Writebol.
"Even as he battles to survive Ebola, this heroic doctor is still focused on the well-being of others," said the statement.
It also noted that Brantly, 33, had been given a unit of blood from a 14-year-old boy who survived Ebola because of his care.
"The young boy and his family wanted to be able to help the doctor who saved his life," it said.
The current outbreak of Ebola is the largest in history and has killed 729 people in Sierra Leone, Guinea, Nigeria and Liberia, the World Health Organization said.
The virus can be fatal in up to 90 percent of cases, though this outbreak has killed about 60 percent of those infected.
On Tuesday, the outbreak claimed the life of Umar Khan, 43, a top doctor in Sierra Leone and the west African nation's sole virologist.