Pro-life campaigners are angered by the appointment of Alison Britton, the co-author of The Case For Assisted Suicide, to her position as the committee's only adviser and sought assurances that she will be impartial.
If passed by the parliament, MacDonald's highly emotive bill could see terminally ill or permanently physically incapable people deciding to end their lives legally.
Britton will advise a parliamentary committee that was specially set up to deal with the ethical arguments.
Gordon Macdonald of the pressure group Care Not Killing has written to Ross Finnie, the Lib Dem MSP who chairs the End of Life Assistance (Scotland) Bill Committee, to express his concerns.
"Alison Britton co-authored with Prof Sheila McLean, a book entitled The Case For Physician Assisted Suicide and a report titled Sometimes a Small Victory," Macdonald said.
"In both these publications they advocate the legalisation of assisted suicide and prioritise patient autonomy over other factors. Moreover, their original research was sponsored by the Voluntary Euthanasia Society for Scotland.
Britton was unavailable for comment. But Finnie emphasised that her appointment was within Holyrood's rules.
He said: "The committee is confident that Ms Britton is in a position to provide impartial advice."
MacDonald said: "I don't know this lady, but I do know Ross Finnie and I have every faith in him."