A leading Tokyo bank today announced plans to buy two commercial lending units of Walter E. Heller International Corp. for $425 million, the largest takeover of a U.S. company ever undertaken by a Japanese firm.

The cash purchase by The Fuji Bank Ltd. marks both the largest U.S. acquisition by a Japanese firm and the first major entry into the domestic financial services market by a Japanese bank. In addition, it effectively terminated a two-week-old $400 million bid for the same two Heller companies by Security Pacific Corp., a Los Angeles bank holding company.

The Fuji bid requires approval by the Federal Reserve Board and Heller shareholders and directors. As a result, both companies do not expect the deal to be completed until considerably later in the year.

With assets of about $80 billion, Fuji ranks as the second largest bank in Japan and the 13th largest in the world, the bank's U.S. representatives said. In terms of assets, it is larger than all but BankAmerica Corp. and Citicorp in the United States.

Yoshiro Araki, Fuji's president, said in a statement that the acqusition, combined with a variety of affiliated Fuji operations in the United States, "is part of Fuji's commitment to provide financial services to the American business marketplace." Araki said the bank expects the finance companies to "operate separately and autonomously."

In an interview, Ko Uemura, Fuji's senior managing director, said the "U.S. market has been and will be most important to Japanese bankers." Uemura emphasized that, while Fuji is involved in major corporate banking activities in the United States, "what we lack completely is contact" with middle-sized U.S. businesses.

Uemura said Fuji does not require approval of the Japanese government to make the purchase, although the minister of finance had been notified of Fuji's plans. He added, however, that only a recent loosening of banking regulation in Japan permitted them to move ahead with the deal.

He said Fuji currently is not interested in becoming involved in retail or consumer banking in the United States, but said Heller's background in "asset-based financing" is an unfamiliar expertise to Japanese bankers.

The two subsidiaries, Walter E. Heller & Co. and Walter E. Heller Overseas Corp., have assets of about $3 billion, about half the parent company's total, a figure that makes it the 19th largest diversified financial company in the United States.

Through those units, the 63-year-old Chicago-based parent company is a major force in the factoring business in which credit companies buy a firm's accounts receivable at a discount in return for assuming a degree of lending risk. The companies have outstanding receivables and loans totaling about $4 billion. Moreover, they are major lenders to middle-sized companies.

The proposed purchase does not, however, apply to Heller's third major subsidiary, American National Bank and Trust Co. of Chicago, that city's sixth largest bank.

Much of the commercial lending field has been in a difficult position in recent years, and Heller International reported a $7 million fourth-quarter loss last year. The parent company also wrote off $78 million in pre-tax earnings last year because of "irregularities" at a regional office.

Thomas Rosencrants, a banking analyst with Chicago's William Blair & Co., said it appeared the Heller companies were likely to operate with more independence under Fuji management than under the umbrella of Security Pacific. An official of the California company had no comment on the announcement.

Although extremely active in setting up domestic and international joint ventures, such as the recent General Motors-Toyota agreement, Japanese firms have generally shied away from acquisitions. Until this Fuji deal, no Japanese company had purchased a U.S. firm for more than about $25 million, according to an executive of Morgan Stanley Co., the investment banking firm that handled this purchase.