Saks Fifth Avenue, one of America's most prestigious retailing names, would become a Japanese-American hybrid under a buy-out plan proposed Thursday by Saks managers and Japan's Tobu Department Store Co.

The prestigious New York-based retailer said its management and the Tokyo retailing giant did not disclose the terms of the offer to Britain's B.A.T. Industries PLC for the 46-store retail chain.Saks said the bid by its management and Tobu is fully financed.

If the bid succeeded, Saks said Tobu would develop Saks stores in Japan and elsewhere in Asia. It said Tobu's new flagship department store in Tokyo, which will have a total sales area of 893,428 square feet, could have a ''substantial portion" set aside for a Saks retail location.

Analysts have speculated that Saks could command about $1 billion. But skepticism has grown, with the U.S. retailing industry traumatized by the bankruptcy filing of the vast Campeau Corp. U.S. department stores and the collapse of the junk bond market.

The involvement of a Japanese buyer in a high-profile U.S. takeover follows the acquisition last year by Mitsubishi Real Estate Group of Rockefeller Center, just steps from Saks' Manhattan flagship store.

But any controversy over Japan's purchase of another American institution is likely to be at least partially defused because the Saks management would operate stores in the United States and Tobu would expand Saks into Asia.

B.A.T. announced last year that it was interested in selling off its retailing properties. The company launched a restructuring when it faced a $21 billion takeover threat from an investment group led by Sir James Goldsmith.

Among those said to be pursuing the prestigious store chain was Henry Kravis, married to dress designer Carolyn Roehm. His Kohlberg Kravis Roberts & Co. headed the $25 billion leveraged buy-out, the biggest ever, of RJR Nabisco last year.

Junk bonds, used to finance many takeovers in recent years, dropped sharply in value last year as doubts grew over Campeau and other junk bond issuers. R.H. Macy & Co., taken private in a management-led leveraged buy-out in 1986, this week reported mounting losses that it attributed to steep price cuts by competitors to generate cash for debt payments.

The market was further damaged by the failure earlier this year of the No. 1 player in the junk bond financings, Drexel Burnham Lambert Inc.