He fed rats GMOs, in the form of Monsanto’s Roundup Ready corn, and they developed tumors. Some died. The study was published in the journal, Food and Chemical Toxicology (wikipedia). Pictures of the rats were published.
A wave of biotech-industry criticism ensued. Pressure built. “Experts” said the study was grossly unscientific, its methods were unprofessional, and Seralini was biased against GMOs from the get-go. Monsanto didn’t like Seralini at all.
The journal which published the Seralini study caved in and retracted it.