T.Rowe Price Associates, Inc., the big Baltimore-based investment adviser, and Robert Fleming Holdings Ltd., of London, today announced plans for a joint venture to manage international investments for American institutional investors such as pension funds.

U.S. Institutional investors are growing increasingly interested in disversifying their portfolios to include stocks of foreign companies , in large part because of the dismal performance of the American stock market in recent years.

For example, one of the first, and still few, companies to establish specialized subsidiaries to manage international investments for U.S. investors, J.P.Morgan Co., Inc., said today that its international Equities Fund has returned an average of 28.1 percent a year since it was established in early 1974.

J.P.Morgan's Morgan Guaranty Trust Co.managed $422 million in international investments for its institutional clients.

T.Rowe Price Vice President M.David Testa said in a news conference today that U.S. private pension funds have about $120 billion invested in stocks, nearly all of them shares of U.S. companies.

He said a number of his firm's clients had asked about investing some parts of their assets in foreign companies.

Testa and Fleming director Charles Nunnely said the joint venture would combine Price's understanding of the needs of U.S. investors with the London firm's broad international research capability.

Each company now manages about $6 billion in client assets.

The day-to-day investment activities will be carried out in London, while the marketing and servicing operation will be done out of Baltimore.

At firest, Nunnely said, the new company, Rowe Price-Fleming International Inc, will aim at managing investments for big individual pension funds. Later, however, the company will form an investment pool, comingling investments of many pension funds.

That arrangement would be similar to a mutual fund, in which individual investors own a certain number of shares of an investment company which in turn invests in shares of a broad number of companies.

J.P. Morgan runs such a "comingled" investment fund now in its international operation.

Nunnely said the firm expects to file incorporation papers in Maryland within a few weeks and to register as an investment advisor with the Securities and Exchange Commission shortly thereafter. It takes 45 days for the SEC to approve an application of an investment advisor. Wednesday the SEC gave tentative approval of the new firm.

Nunnely said his firm had been interested in doing international investment management for U.S. pension funds for some time, but felt itself incapable of dealing adequately with U.S. needs.

Testa said Rowe Price felt the need for solid research on individual companies in foreign countries, similar to the kind of research it does on American companies.

Both officials said the companies had spent about a year working out the details of the new company, in part because of the SEC's concern about the area.

Testa said that while the investment policy of the proposed company has not yet been worked out, he expects a major portion of the firm's investment will be in Japan and the Far East and, to a lesser extent, in western Europe.

He said that "our strong expectation is that we'll be managing a couple hundred million dollars of assets within a few years."