THE SOVIET government "emphatically re jects," Andrei Groymko told the United Nations Tuesday, "the absurd talk" about the use of chemical weapons--by Soviet clients in Laos and Cambodia and by Moscow itself in Afghanistan. Yesterday President Reagan responded, telling the same audience that the Soviet Union had broken its treaty word not to use chemical and biological weapons.

So who is telling the truth? The United Nations special session on disarmament can hardly dodge the question if it is at all serious about its high mission. Even if each superpower were not demanding that the conference validate its position on the CW issue, the conferees would have a deep interest in establishing their relative good faith. Mr. Gromyko uttered his denial, after all, in the context of proposing that yet another international agreement banning chemical weapons be made. Surely the United Nations special session will find it relevant to note how the Soviet government has recently been treating its earlier international pledges on this very matter.

Mr. Reagan asserted that the United States has "conclusive evidence" of the use of chemical and toxin weapons by the Soviets in Afghanistan and by the Vietnamese and Cambodian regimes in Laos and Cambodia. We presume the United Nations conferees will want to inspect the American evidence. But, wisely, Mr. Reagan did not let it go at that. The Communist perpetrators of chemical warfare have so far denied United Nations investigators access to the regions they have despoiled. Mr. Reagan called on them to admit those U.N. experts so that they can "conduct an effective, independent investigation to verify cessation of these horrors."

It will be said by some in the great pipe organ on the East River that Mr. Reagan was simply making anti-communist propaganda. Why deny it? He was making propaganda, the best kind and a necessary kind. What he said was true and deserved to be said precisely in that forum, vulnerable as it is, we fear, to windy one-sided expostulations. The United Nations disarmament conference badly needs to come to terms with a country, the Soviet Union, with the gall to demand a new ban on an odious form of warfare that it is practicing and covering up even as it speaks.