As a plaintiff, in 1969, Monsanto sued Rohm and Haas for infringement of Monsanto's patent for the herbicide propanil. In Monsanto Co. v. Rohm and Haas Co., the Third Circuit Court of Appeals ruled against Monsanto on the basis that the company had fraudulently procured the patent it sought to enforce.
Monsanto argues, the principle behind a farmer’s seed contract is simple: a business must be paid for its product., but that a very small percentage of farmers do not honor this agreement. While many lawsuits involve breach of Monsanto's Technology Agreement, farmers who have not signed this type of contract, but do use the patented seed, can also be found liable for violating Monsanto's patent. Monsanto has stated it will not "exercise its patent rights where trace amounts of our patented seed or traits are present in farmer's fields as a result of inadvertent means." However, history shows this is not true as Monsanto has sued farmers, and won, when pollen from nearby GMO seed mixed with the farmer's plants, the seed of which had some patented genes. These farmers had to sell their whole farm to pay fines, and court costs.
Monsanto has also been sued for effects from Agent Orange, pollution from dioxin, polychlorinate biphenyls (PCBs), alachlor (a common pesticide), and dicamba.
Source
Wikipedia.