WADA director general David Howman would not elaborate on what he wrote in the letter sent to the ATP, but he told The Associated Press he hopes it "would bring a considered response."
"Our task is to protect the clean athletes and to make sure that these sorts of things don't recur," Howman said by telephone. "And if we didn't take any steps, somebody would be knocking on our door saying, 'Well, what are you doing about this?"'
Agassi wrote in his soon-to-be-released autobiography "Open" that he ingested crystal meth and then lied to the governing body of men's tennis to avoid a suspension after failing a doping test.
Howman said the letter was specifically addressed to the ATP, but the International Tennis Federation would be made aware of it.
"The ATP can confirm it has received a letter from WADA," the tour said in a statement e-mailed to the AP yesterday. "When it responds it will do so directly to WADA and not through the media."
The statement continued, in part: "The ATP would also like to reiterate its policy of not commenting on anti-doping test results unless and until an anti-doping violation has been found."
That was the crux of what the ATP said last week, when excerpts from the book revealed that eight-time Grand Slam champion Agassi admitted he used crystal meth in 1997 and said he had wriggled his way out of a suspension after a positive drug test that year.
Other tennis and doping authorities initially expressed disappointment at those revelations, but they also said it was too late for sanctions because of an eight-year limitation rule.
Howman, however, has urged the ATP to look more closely into the situation and inform WADA of its findings.
He wants his group to "respond in as responsible fashion as possible by making sure we don't start preaching or teaching before we have all the relevant information. Once we've got the relevant information we can make better judgment calls."
Agassi, who is married to tennis great Steffi Graf, is a former top-ranked player who won all four Grand Slam titles.
He has also raised tens of millions of dollars for at-risk youths in his hometown of Las Vegas and opened a preparatory academy there.
Besides admitting to using crystal meth in the book, Agassi also wrote that he swallowed a pill given to him by his father - apparently when he was a junior player - that may have been the amphetamine speed.
"These things of yesteryear, before our time, are such that we're required to investigate them but we don't have the tools with which we can manage unless there is something that comes from such an investigation," Howman said of WADA, which was founded in 1999.
Howman said he expected the ATP to proceed with caution. "We've got to be reasonably fair and give them time, he said.