Kohlberg, Kravis, Roberts Inc., a private New York investment firm, has offered $90 a share, or $20.3 billion, in cash and securities for RJR Nabisco Inc., the Atlanta-based food and tobacco giant.

If completed, the bid would be the largest takeover in history.Kohlberg said it would begin a tender offer some time this week for 87 percent of RJR Nabisco's 225.3 million shares outstanding. The tender offer would consist of unspecified proportions of cash and unspecified securities.

If the tender offer were successful, holders of the remaining 13 percent of RJR Nabisco's stock would receive securities worth $90 a share, said Ruth Pachman, a Kohlberg spokesman.

The bid tops another potential offer disclosed Thursday when a group led by RJR Nabisco's top management said it was considering offering $75 a share, or $16.9 billion, for the company.

Either transaction, if completed, would be the biggest corporate acquisition ever, exceeding Chevron Corp.'s $13.3 billion merger with Gulf Oil Corp. in 1984.

RJR Nabisco spokeswoman Polly Howes said the company had no comment on the Kohlberg, Kravis offer.

RJR Nabisco's stock price jumped $9.375 to $86.125 in heavy midday trading on the New York Stock Exchange, where it was the most active NYSE-listed issue.

On Thursday, RJR Nabisco chief executive F. Ross Johnson and other members of top management, backed by financial services giant Shearson Lehman Hutton Inc., said they were considering making a $75-a-share bid for RJR Nabisco, the 19th-largest U.S. industrial concern.

After the announcement, RJR Na-bisco's stock rocketed more than $21 to over $77 a share, suggesting Wall Street believe another, higher offer for the company would be forthcoming.

RJR Nabisco, which was formed by the 1985 merger of R.J. Reynolds Industries and Nabisco Brands Inc., makes Camel and Winston cigarettes, Ritz crackers and Oreo cookies and Del Monte canned fruits and vegetables.

The RJR Nabisco bid is part of a wave of takeover moves involving food companies.