In September 1969, they set up a Trial Research Group, which included law professors, medical researchers (including Harada), sociologists and even Michiko Ishimure to provide useful material to the lawyers to improve their legal arguments. Their report, Corporate Responsibility for Minamata Disease: Chissos Illegal Acts, published in August 1970, formed the basis of the ultimately successful lawsuit.The trial lasted almost four years. The litigation groups lawyers sought to prove Chissos corporate negligence. Three main legal points had to be overcome to win the case. First, the lawyers had to show that methylmercury caused Minamata disease and that the companys factory was the source of pollution. The extensive research by Kumadai and the governments conclusion meant that this point was proved quite easily. Second, they needed to show that Chisso could and should have anticipated the effect of its wastewater and taken steps to prevent the tragedy (i.e., was the company negligent in its duty of care). Third, it had to disprove that the "sympathy money" agreement of 1959, which forbade the patients from claiming any further compensation, was a legally binding contract. The trial heard from patients and their families, but the most important testimony came from Chisso executives and employees. The most dramatic testimony came from Hosokawa, who spoke on 4 July 1970 from his hospital bed where he was dying of cancer. Hosokawa explained his experiments with cats, including the infamous "cat 400", which developed Minamata disease after being fed factory wastewater.
The immediate effect was the death of fish at the mouth of the river, and from that point on, new Minamata disease victims began to appear in other fishing villages up and down the coast of the Shiranui Sea, as the pollution spread over an even greater area.Chisso failed to co-operate with the Kumadai research team. It withheld information on its industrial processes, leaving researchers to speculate what products the factory was producing and by what methods. The Chisso factorys hospital director, Hajime Hosokawa, established a laboratory in the research division of the facility to carry out his own experiments into Minamata disease in July 1959. Food to which factory wastewater had been added was fed to healthy cats. Seventy-eight days into the experiment, cat 400 exhibited symptoms of Minamata disease and pathological examinations confirmed a diagnosis of organic mercury poisoning. Chisso did not reveal these significant results to the investigators and ordered Hosokawa to stop his research.In an attempt to undermine Kumadai researchers organic mercury theory, Chisso and other parties with a vested interest that the factory remain open (including the Ministry of International Trade and Industry and the Japan Chemical Industry Association) funded research into alternative causes of the disease, other than its own waste. Compensation of fishermen and patients, 1959
Polluting wastewater had damaged the fisheries around Minamata ever since the opening of the Chisso factory in 1908.