TWO MONTHS AGO, Secretary of State Alexander Haig, made headlines with the charge that while the United States is often judged by a "super-critical standard," the Soviet Union was conducting chemical and biological warfare in Southeast Asia without public knowledge or complaint. The charge dealt specifically with the use of deadly trichothecene toxins produced by a type of fungus.

The evidence at that time was fairly flimsy. The mycotoxins had been found in only a single sample of leaf and stem. Crucial "negative controls," showing that the substances were not present in areas that had not been attacked, were unaccountably missing. Statements that this fungus is not naturally found in Southeast Asia were premature and, as it turns out, incorrect.

Now, however, the missing evidence has been supplied. The government announced yesterday that a combination of four mycotoxins, all from different strains of the fungus, has been found in samples taken from both Laos and Kampuchea. The samples include water and scrapings from rocks, in addition to vegetation. The negative controls confirm the accuracy of the chemical tests. The mixture of four different toxins provides additional confirmation that the findings are not the result of even the most bizarre natural outbreak. As Assistant Secretary of State Richard Burt told the Senate Foreign Relations Committee, "The significance of this discovery . . . can be simply stated. We had solved the mystery. We had fitted together the jigsaw puzzle which had bedeviled us for five years."

The evidence that a biological weapon is being used now, therefore, seems solid. There is no comparably firm evidence tying responsibility for its use to the Soviet Union. But there is very strong suggestive evidence. Though the toxins have reportedly been sprayed by local forces, Soviet chemical warfare experts are known to be in the area. Facilities capable of mass-producing the fungus and extracting the toxin are known to exist in the Soviet Union, some, according to Mr. Burt "under military control and with heavy military guard." No such facilities are known to exist in Southeast Asia.

It seems reasonable to conclude that the Soviet Union is directing a vicious campaign of chemical and biological warfare in one of the most remote areas of the world against people unable to protect themselves or understand what is happening to them. This warfare, moreover, is in direct violation of international agreements signed by the Soviet Union. The government's information merits the most serious possible international inquiry to prove or disprove the charges and to hold the Soviet Union accountable for a flagrant violation of an arms control agreement.

As the administration correctly emphasizes, this must not be allowed to be seen as a U.S.-Soviet confrontation. The United Nations committee of inquiry is therefore the correct body to bring the investigation to a prompt conclusion. The United States' job is to make it plain that this Soviet warfare is an attack on the integrity of all nations and international agreements, as well as on, in Franklin Roosevelt's words, "the general opinion of civilized mankind."