Proof of Indochina poison elusive

The United States has suffered a setback in its efforts to gain international acceptance of charges that the Soviet Union and Vietnam have been using chemical and biological weapons in Indochina, Monitor writer Frederic Moritz reports.

Australian scientists are reported to have found no traces of the mycotoxin known as ''yellow rain'' - nor of any other poison - on leaves and pebbles said to have been collected by anticommunist refugees in Laos.

An Australian official picked up the samples last April at the Ban Vinai refugee camp in Thailand. Refugees in the camp said they found the samples after Vietnamese chemical attacks in the area.

The US says the specimens it obtained from western Kampuchea (Cambodia) show definite traces of the ''yellow rain'' biological weapon known as mycotoxins (poisons produced by fungi). But it adds that the problem of getting proof from Laos is more difficult because samples collected by often unqualified refugees must be carried longer distances for analysis.

Both supporters and critics of the US charges agree that a major obstacle in confirming or disproving the charges has been the problem of accurately retrieving and quickly transporting reliable battlefield samples to the laboratory from distant areas of conflict. THe US hope has been that if other countries, such as Australia, join the search for answers, the American charges will eventually gain in credibility.