The State Department will make public today what it believes is the strongest evidence yet in its nine-month campaign to prove that mycotoxin poisons have been used in chemical warfare in Southeast Asia.

After an apparent poison gas attack in Cambodia on Feb. 13, an American doctor took blood samples from two Khmer Rouge guerrillas who were victims of the attack.

The blood samples were flown to the United States and analyzed. The results to be announced today are that the blood contains high levels of two mycotoxin poisons called T2 and HT2, according to Dr. Sharon Watson, a toxicologist with the Army Medical Intelligence and Information Agency.

The new evidence of blood samples "provides conclusive proof of recent exposure to trichothecenes mycotoxins ," according to a briefing paper from the Army laboratory that oversaw the analysis. "The unusually high body fluid levels of toxin (up to 22 parts per billion) indicate exposure to a large dose of toxin."

Critics conceded that the evidence helps the government's case. They added, however, that the government should now offer more evidence to counter the possibility that the Khmer Rouge soldiers were poisoned some other way and then concocted a story about the gas attack to gain sympathy for their cause.

The new sample is the sixth offered publicly by the State Department but is the first to show that someone who claimed to have been under chemical attack actually had the poisons in his body. Other samples of the toxin were taken from the soil and vegetation.

The State Department charges that the Soviets are responsible for the use of chemical weapons in Asia and Afghanistan, though in Southeast Asia they are actually used by Vietnamese or Laotian troops.

The United States has begun a worldwide campaign to convince other countries and persuade them to openly recognize the U.S. evidence of chemical war.

The United States has also given out special sampling kits to people in Southeast Asia to help them continue collecting samples of the poison, commonly known as "yellow rain."

According to Watson and other sources, the attack that produced the new blood samples took place at the village of Tuol Chrey in Cambodia, where about 17 Khmer Rouge guerrillas were shelled by the Vietnamese with captured American 105-mm. artillery rounds.

One or more of the shells contained poison gas, a leader of the Khmer unit named Prak Reth told Watson and William Branigin of the Washington Post. The guerrillas were about 150 yards away from where the shells hit.

They soon suffered severe symptoms of toxic poisoning, including severe eye irritation, severe bloody vomiting that continued for more than a day, bloody diarrhea, trembling, and difficulty in breathing.

Within a day the guerrillas sought treatment in the Phum Tmey camp, not far from the site of the attack. There Dr. Amos Townsend, a retired U.S. Air Force colonel and now a physician with the International Rescue Committee, took blood and urine samples from Prak Reth and another guerrilla.

Watson said that samples from people of the same area, age, and background also were taken as controls. Samples of vegetation, dirt, corn, rice, and water were also taken from the area of the attack to check against any possible natural occurrences of trichothecenes, poisons produced by a fungus that attacks plants and stored grain. No toxin was found in the controls.

Scientists are uncertain as to whether any fungus native to Southeast Asia produces the toxins found in these samples. It is common in the colder climates of the Soviet Union and Upper Midwest states such as Wisconsin, scientists say.

The blood samples were analyzed by Dr. Chester Mirocha of the University of Minnesota, an expert on trichothecene poisons. He said that Prak Reth showed a level of 22 parts per billion of one trichothecene called T2 in his blood. When this toxin enters the system, it is believed to break down into another toxin called HT2. Ten parts per billion of HT2 were found in his blood.

The other guerrilla sampled had 11 parts per billion of T2 and 10 parts per billion of HT2 in his blood.

A dose of 1 to 5 parts per billion can kill animals, according to Mirocha.

Watson said the toxin apparently is made from a crude extract of poisonous fungi, then mixed with a solvent to speed the passage of the poison through the skin and into the body.

Another 50 to 100 samples and control samples from Asia are being analyzed in laboratories around the country. A scientific paper giving details of all the work to date will be presented for the first time at a meeting of toxicologists in Vienna in September.