The trial opened in Italy today of five senior executives accused of responsibility for the 1976 chemical plant explosion in Seveso, Italy, but it is not likely to solve the biggest mystery about the disaster--the whereabouts of 2.2 tons of highly toxic dioxin waste from the explosion.

The waste, made up of small whitish crystals that are said to be 10,000 times more dangerous than cyanide, was collected in 41 steel drums and loaded onto a truck that left Seveso on Sept. 10 last year. It was taken in utmost secrecy to a storage site in the northeastern French town of Saint-Quentin where it was transferred to another truck.

What became of it then has so far proved impossible to establish, despite an intensive investigation lasting several months by governments, environmental groups and the media throughout Western Europe.

The mystery has again made the Swiss pharmaceutical company of Hoffman-La Roche, which owned the Seveso factory through a subsidiary, Givaudan, a target of public anger. Last weekend, an unsuccessful attempt was made to explode a bomb outside the company's Paris headquarters. Hoffman-La Roche officials say they arranged for the waste to be removed by a company called Mannesmann Italiana that in turn subcontracted the job to a French disposal expert, Bernard Paringaud.

One of the conditions of this elaborate arrangement was that the final destination of the dioxin remain secret "for psychological reasons.".

The one person who presumably knows something is Paringaud, a former French parachute officer who has made a successful business out of getting rid of other people's embarrassing rubbish. But he is refusing to talk, even though he faces a possible two-year prison sentence for false customs declarations.

Today's court hearing over the Seveso affair in the north Italian town of Monza was adjourned after five hours. None of the five defendants, who include the Swiss chairman of the Seveso Icmesa plant where the explosion occurred, was present.