The trial in Paris over the January 2015 Islamist attacks on the Charlie Hebdo satirical weekly and a Jewish supermarket was suspended for one day Thursday after one of the accused fell ill in the box.
The defendant, Nezar Mickael Pastor Alwatik, will undergo diagnostic tests including for Covid-19, his lawyer said.
Judge Regis de Jorna announced in court that the trial was being suspended until Friday at 9:30 am (0730 GMT) to allow time for the medical exams.
Pastor Alwatik's lawyer Marie Dose said her client had been suffering from "a lot of fever, coughing, vomiting and headaches".
She said such symptoms were worrying in the light of the Covid-19 pandemic and warned that hearings should not even resume Friday if his Covid-19 test result was not yet in.
The defendant was taken to the Fleury-Merogis prison outside Paris for the tests.
Fourteen suspects went on trial on September 2, accused of being accomplices after the perpetrators were killed in the wake of the massacres.
Twelve people, including some of France's most celebrated cartoonists, were killed in the attack on Charlie Hebdo.
A female police officer was killed a day later and a day after that four men were killed in a hostage taking at a Jewish supermarket