New Zealand troops who served in the Vietnam War suffered significant genetic damage from exposure to Agent Orange, a study suggests.
Agent Orange has been linked to disabilities in children
The chemical was used by the US military in Vietnam in the 1960s.
It has been blamed for a variety of medical conditions suffered by soldiers and up to four million Vietnamese.
The study by New Zealand's scientists could have a big effect on campaigners' efforts to sue major chemical firms and the US government, correspondents say.
The US military sprayed some 80m litres of Agent Orange on North and South Vietnam.
The aim was to destroy jungle foliage in order to find communist fighters more easily.
Agent Orange contained highly toxic dioxins which have since been blamed for causing cancers and other illnesses.
They have also been blamed for birth defects suffered by the children and even grandchildren of Vietnam veterans and Vietnamese civilians.
This has been strongly contested by the two main companies which made it - Dow and Monsanto - and the US government, the BBC's Bill Hayton in Hanoi says.
A team from New Zealand's Massey University has now shown that the group of 24 Vietnam veterans it tested suffered significant genetic damage, compared with a similar sized group of soldiers who did not serve in Vietnam, our correspondent says.
This may be crucial evidence in the lengthy legal battles still being waged in courts in the US and other countries to prove or disprove the link between Agent Orange and a legacy of illness across three continents, our correspondent says.