The king, who was taken to Siriraj hospital in Bangkok last night, appeared to be improving.
"On Saturday morning, his medical team announced that after being administered antibiotics intravenously, his blood pressure stabilised and his fever dropped," the palace said in a statement.
The statement said doctors had performed further examination with special equipment to try to determine the cause of blood infection for further treatment.
Bhumibol, who has suffered from a range of ailments in recent years, is treated as a near-deity in Thailand and his health is a subject of public concern.
Bhumibol is a member of the Chakri dynasty, which has occupied the Thai throne since the 18th century.
The US-born and Swiss-educated king ascended to the throne in 1946 and has reigned over more than 20 prime ministers, 17 military coups and 17 constitutions.