McGraw-Hill's board of directors yesterday angrily rejected a merger proposal from American Express and vowed to "take all appropriate actions" to stop the takeover attempt.

In a lengthy, acid letter to thedirectors of the credit card company, Harold McGraw, chairman of McGraw-Hill, said the proposed merger threatened the "independence and credibility" of his company, which publishes business Week and 60 other magazines, and also raises disturbing antitrust and conflict-of-interest issues.

Upset, too, over the manner in which the proposal was made, McGraw further charged Roger H. Morley, the American express president who sat quietly as a director on McGraw-Hill's board while his company planned a takeover, with "misappropriating" confidential confidential information. He said Morley's actions amounted to a "breach of trust and conspiracy to subvert the interest of McGraw-Hill and its shareholders."

In an ovious swipe at the 42-year-old chairman of American Express, James D. Robinson III, McGraw said the situation might be explained as "impulsive, precipitous and immature actions taken by yoiunger members of management before the more experienced members of your board had ample opportunity to fully consider this reckless proposal and all of its implications to each company."

"The background and manner of your proposal demonstrates that American Express lacks the integrity, corporate morality and sensitivity to professional responsibility essetial to the McGraw-Hill publishing, broadcasting and credit and credit rating services."

Stunned by the tone of McGraw's remarks, Robinson announced his company would nevertheless proceed with plans to acqurie the publishing company. He called McGraw's attacks "unwarranted, unfounded" and accused the publisher of attempting "to confuse the issues on personal and irrelevant gropunds rather than give McGraw-Hill shareholders an opportunity to decide for themselves."

The action uesterday sets the stage for a variety of corporate and courtroom maneuvers. American Express, which currently does not own any McGaw-Hill stock, may decide to raise its bid or make an decide to raise enough shares to win a controlling interest.

Since American Express' Proposal last Tuesday to offer $34 a share for the 24.1 million shares of McGraw-Hill, the price of the stock has climbed from $26 to $33.50. The stock did nt trade yesterday.

Whatever formal offer American Express puts forth, it still must file with the Securities and Exchange Commission for approval, with the New York Attorney General's office for clearance, with the Federal Trade Commission for review of possible antitrust violations and with the Federal Communications Commission for the exchange of licenses for the four television stations owned by McGraw-Hill.

McGraw-Hill is likely to challenge American Express in all these forums. It also threatened legal action of its own yesterday against American Express, Morley and the 21 prestigious members of the American Express Board.

Even Morgan Guranty Trust Co. of New York, McGraw-Hill's principal bank, could become embroiled in such litigation. McGraw yesterday blasted American Express for obtaining financing for its takeover from the bank.

Robinson, in his statement, said American Espress had considered all legal matters before proceeding with its proposal to discuss a merger and stated he was "comfortable tha the proposal is legal and proper."

Also, prior to the decision by the McGraw-Hill board, Robinson said he had hand-deilvered to Harold McGraw a letter specifically addressing the independence issue. In the letter, Robinson reportedly stated he was sympathetic with McGraw's concern for editorial freedom and promised to maintain the publishing company's independence if a merger took place.

McGraw made no mention of the letter during his press conference. Asked, too, about rumors that otehr members of the McGraw family might be willing to sell their holdings in the company at the right place, McGraw said, "There is no dissension in the family." The family owns about 20 percent of McGraw-Hill's stock.

The bid by American Express marks the fourth major acquisition effort by the cash-rich company in recent months. The three other attempts -- against the Book-of-the-Month Club. Philiadelphia Life Insurance and Walt Disney Productions -- all failed.

Wall Street analysts have viewed the Out-of-the-blue proposal as a favorable one for American Express, arguing that it would give the company some attractive diversity when its profitable credit card and travelers checks businesses are facing more intense competition from leading banks, In addition, a merger with McGraw-Hill would provide a helathy shot of operating revenues to balance the investment revenues earned on the company's check float and Firemen's Fund insurance subsidiary.