In their Aug. 21 column, ''A Mexican Journalist's Fatal Scoop,'' Jack Anderson and Dale Van Atta use the testimony of Laurence Victor Harrison, a DEA informant at a U.S. District Court in Los Angeles to speculate on alleged events in Mexico in 1984 and 1985.

A press release issued last July 10 by the Attorney General's Office in Mexico exposed the inconsistencies in Mr. Harrison's testimony and considered it a false and irresponsible statement. A similar conclusion was reached by the U.S. District judge in Los Angeles.

A July 7 Post article {''Judge Discredits Testimony in Mamarena Case''} clearly reports:

"The judge in the Enrique 'Kiki' Camarena murder trial today called 'incompetent' the testimony of a prosecution witness who had alleged that Mexican drug traffickers were linked with the Central Intelligence Agency and were helping to arm the Nicaraguan contra rebels.

"U.S. District Judge Edward Rafeedie, ruled that Laurence Victor Harrison, who made the allegations, could not be questioned in front of jurors because his 'testimony is based on hearsay, gossip and speculation.' "

I believe that elaborations based on such a discredited testimony suffer from the same lack of credibility and can only be used to misinform readers. ANTONIO OCARANZA Press Attache, Embassy of Mexico Washington