Jyoti Basu has been hospitalised since Jan 1 following a pneumonia attack.
"He is stable with ventilator support. His condition is the same as it was late Wednesday, Jan 6," AMRI hospital Medical Superintendent Debasish Sharma was quoted by news agency as saying.
CPI-M politburo member Sitaram Yechuri said that 96-year-old leader was responding to a new course of antibiotics.
"He has been a fighter all his life. And now also he is fighting," Yechuri said.
Yechuri said lot of disinformation was spread on Wednesday, Jan 6, about Basu's condition.
"All this disinformation (being spread) should stop. We have to rely on the information the doctors are providing," he added.
Prime Minister Manmohan Singh was slated to call on the ailing leader around noon on Thursday, Jan 7.