The World Health Organisation's cancer agency advised the academic experts not to disclose documents they were asked to release under United States freedom of information laws.
IARC wrote to scientists on its review panel: "IARC is the sole owner of such materials. IARC requests you and your institute not to release any (such) documents."
The row follows a long-running argument over reviews into glyphosate.
Monsanto vice president of strategy Scott Partridge said:"This latest disclosure of IARC's internal correspondence with the U.S. government shows the extent they will go in order to hide their process from US officials, regulators, and the public.
"It's ridiculous that IARC would ask US governmental agencies and other public institutions to hide public documents. The public deserves a process that is guided by sound science, not IARC's secret agendas."