Helen Steel, unemployed, and Dave Morris, a single parent living on state benefits, say they did not publish the leaflet but its comments are true or fair comment on a matter of public interest. They have no legal aid and are defending themselves.
The leaflet suggested that eating McDonald's food could cause bowel and breast cancer or heart disease and that the production of that food caused hunger in Third World countries and destruction of rainforests. It also said the restaurant chain's employees were exploited, ill-paid and would be dismissed if they attempted to join trade unions.
The costs of the hearing, expected to last three months and involving dozens of expert witnesses, are expected to top pounds 1m. Should they lose, the defendants have neither the money nor assets to make the slightest dent in that figure but McDonald's is determined to clear its name.
The courts have ruled that the case should be heard without a jury because of the complexity of the issues and the mass of scientific evidence involved.
Opening the case for McDonald's, Richard Rampton QC, said distributing the leaflet outside McDonald's restaurants and its headquarters in East Finchley, London, was 'a wholesale attack on almost every aspect of the plaintiff's business'.
It contained 'numerous statements which were wholly defamatory and erroneous in every respect'. Free speech was not at issue, McDonald's was simply determined to 'stop the flow of poison' and prevent plainly false statements being made about it.
Mr Morris, 39, and Ms Steel, 28, each have a laptop computer and several 'McKenzie friends' - people with legal knowledge who can prompt and advise but not address the court.
Mr Rampton described the foundation of the company in 1955 by Ray Kroc in Illinois, and its growth to a chain of 14,000 restaurants in 70 nations with a turnover of dollars 23.5bn last year - 'the largest food service organisation in the world'. The UK subsidiary has 530 outlets.
He said the McDonald's corporation, its subsidiaries, franchisees and partners were committed to 'uniformity of excellence' in the quality of food, customer service, well-being of employees - especially women and ethnic minorities - and in their responsibilities to the environment.
He described how, after a demonstration in 1989, the British subsidiary of McDonald's decided to track down the publishers of the leaflet using two firms of private detectives. They pinned this down to London Greenpeace, no relation of the Greenpeace International environmental organisation but a group of anarchists and pacifists intent on attacking multinational corporations and what they called 'the industrial death machine'. Private detectives attended its meetings.
In 1991 letters were sent to five activists demanding they apolog ise in court and promise not to repeat the libel. Three capitulated, but not Mr Morris or Ms Steel.
The case continues.