But following their trip to Thailand, Meselson and Seeley reiterated their previous skepticism over the scientific quality of the State Department's evidence.

This skepticism began with their observations that bee feces bore striking similarities--in size, appearance, and characteristics--to alleged samples of yellow rain, many of which contained pollen.

In various parts of Thailand, the scientists found what they said was the first evidence that Southeast Asian honey bees actually do perform defecation flights, which produce yellow spots similar to what the government has alleged is evidence of yellow rain.

Feces

"People do not recognize these spots for what they are," Meselson said at last week's press conference. "They are in fact the feces of wild honey bees."

Commenting on the State Department's evidence, Meselson said in an interview following the press conference: "They have a lot of evidence, but it is of low quality... They start with a hypothesis, and then they try to collect things that agree with their hypothesis."

Refutation

Referring to samples allegedly containing mycotoxins, he said that scientists have not used proper scientific controls--comparing them with non-contaminated samples--to see if they occur naturally.

Meselson added that because mycotoxins occur naturally in India, he believed they could also appear in Southeast Asia, which has a similar climate.

Meselson expressed similar doubts about the reliability of interviews held with refugees, and he also said that no spent or unspent munition containing toxins has ever been found since reports of yellow rain started seven years ago.

While Meselson has held out for conclusive evidence of chemical warfare, he has drawn criticism from some areas over his conclusions and motives.

Talking Tough

"Continued unsupported speculation on a possible natural explanation of the origin of causalities and deaths from the prohibited use of chemical weapons only serves to divert serious attention from the real issues on chemical warfare," State Department spokesman Alan Romberg told reporters last week.

Another official, who asked not to be identified, called Meselson's allegation that sufficient controls did not been done in the government investigation of samples of leaves, rocks, or blood containing mycotoxins "a bunch of bullshit."

He said that the government "can't manipulate the environment like you can in academic," but had gathered as strong evidence as possible. "We have taken a fairly good number of control samples," he said. "None of these control samples have contained mycotoxins."