Agent Orange was intended to defoliate the jungle
Vietnam is staging "Orange Day" to raise funds for people it says suffered because of the spraying of Agent Orange by US troops during the Vietnam War.
The campaign's organisers hope to raise more than $3m (£1.8m) to help the victims of the dioxin-laced herbicides.
They say Agent Orange was first used by the US in Vietnam 48 years ago.
In March, America's top court let stand the dismissal of lawsuits by Vietnamese nationals and US war veterans seeking damages from the chemical's makers.
Herbicide used to clear vegetation, denying enemy forces cover
Name derives from orange markings on the drums the chemical was shipped in
Children born in areas sprayed have disproportionate rate of mental and physical problems
Hanoi says Agent Orange, named after its orange-striped containers, caused some 400,000 deaths and millions of cases of cancer and other illnesses.
Washington has argued that there is no internationally-accepted scientific evidence linking Agent Orange to birth defects and other illnesses.
The US is currently involved in a project to help clean up dioxin "hot spots" in Vietnam's central city of Danang and is helping fund services to the disabled communities in the country.
The Vietnam War ended in 1975.