PHH Chairman Jerry Geckle is guarding a secret known only to a few top executives at the PHH Group in Hunt Valley, Md. -- a $600 million international company that serves corporations by managing their fleets of cars, helping relocate their employes and flying their executives' planes.

Geckle's secret is the new line of business PHH officials apparently have decided to add as they plan for the company's growth into the 1990s. Geckle says he expects the move "within the next 12 months."

Secrets being secrets, Geckle won't identify the line of business or say what company might be acquired. He does say, with undisguised enthusiasm, that PHH will not go into manufacturing or consumer services but will stay in the corporate service business.

If Geckle, who is completing 30 years with the company, won't reveal his secret, he will at least say why he won't. "It's not to be devious. It's just that from a strategic standpoint, we want to kind of keep it between ourselves until we're ready to go. We're pretty excited about it, and optimistic about it."

Growth by acquisition is not new at PHH, which began life in 1946 as Peterson, Howell and Heather, the last names of its founders. National Truckers Services, offering fuel purchase systems, was acquired in 1969. Three corporate aviation firms were added in 1982, 1983 and 1984 to create PHH Aviation Services, which leases and manages corporate airplanes and provides charter service. Fantus, a commercial site selection firm, was acquired in 1983. And U.S. Mortgage was bought last fall.

The key PHH acquisition was Homequity in 1971, which brought PHH into the employe relocation business. Last October, PHH bought Transamerica Relocation Service for $50 million. The deal has created a $480 million business: $330 million from Homequity and $150 million from Transamerica. The combined staff numbers 1,150, with 900 from Homequity and 250 from Transamerica. Between them, the two firms relocated 25,000 families in fiscal 1984, 18,000 by Homequity, 7,000 by Transamerica.

PHH growth has been rapid and steady. With 226,000 vehicles on the road, PHH is the nation's largest fleet leasing and management company. With the acquisition of Transamerica, Homequity appears to have passed Merrill Lynch as the nation's biggest relocation company. Together, the Peterson group and Homequity, the two main "legs" of the company, account for 88 percent of the revenue. The aircraft business is the third "leg," though much smaller. There is much speculation within PHH and among the Wall Street analysts over PHH's fourth "leg."

The timing of acquisition activity may be affected by the price of PHH stock, which the company hopes to use to acquire a new company. PHH stock went up to 46 1/4 in 1983 and back down to 18 3/8 last June. It has since moved back up, closing Friday at 27 5/8.

Analyst Joel H. Krasner, who follows PHH for Dean Witter, noted that the higher the stock price, the easier it will be for PHH to acquire a company, and that $30 a share seemed to be a key level for the firm to proceed. Krasner said, however, that if PHH issued new stock for acquisition purposes, the dilution of the stock might affect earnings. As a result, he said, the market might not react favorably to an acquisition.

The price of PHH stock slid in 1983 on news that company earnings, which had been growing by an average of 21 percent a year from 1979-'83, would go up only 5 percent in fiscal 1984. The flattening of earnings was caused by tougher price competition in the vehicle leasing business and falling interest rates, which trimmed earnings on investments. In fiscal 1985, ending in April, analysts expect earnings to bounce back and grow by 8 to 10 percent. From the $2.30 per share earned in fiscal 1984, PHH is expected to move up to about $2.50 in 1985 and $2.75 in 1986, according to William M. Legg, transportation analyst at Alex. Brown & Sons Inc. in Baltimore.

Alex. Brown & Sons, the company that took PHH public in 1958, also helped bring PHH onto the London Stock Exchange last month, an event that capped a 10-year effort by PHH to become an established business in the United Kingdom. PHH's British companies have $25 million in sales, divided between fleet management and employe relocation. Geckle, who said he sees major business opportunities in England, predicted sales there will double in five years.

The listing is prestigious for PHH, which has 400 employes in England and is building an office building. Some 10 percent to 12 percent of PHH stock is held by British and Scottish investors, Geckle said. They appear to invest in PHH, he said, for many of the same reasons that U.S. institutions do -- consistent growth, 103 quarters of dividends and an average return on equity of 20.9 percent during the last 10 years. Standard & Poor's, which gives the company an A+ rating, lists 118 institutional investors holding 8.3 million of the 15.8 million shares outstanding.

Alan S. Parsow, the young Nebraska investment fund manager who has been buying big chunks of stock in Evaluation Research Corp. of Virginia and asking for a seat on the board of directors, recently met for the first time with the ERC board of directors at the invitation of President Jack Aalseth. Aalseth said he found Parsow, 34, to be "a very capable young fellow who is worthy of consideration" for a seat on the board. However, Aalseth was unwilling to commit himself.

Parsow said he would be "very much surprised" if he didn't get a seat on the board at the next annual meeting in May. Before Parsow can be formally seated, Aalseth said, Parsow will have to pass a government security clearance. ERC is heavily involved in defense work.

Aalseth said that Parsow brought the ERC board a possible acquisition candidate, a $30 million West Coast high-tech company. Aalseth said he planned to work with Parsow on it. Parsow said that if things went well, the acquisition could be completed in six months.

Legg Mason has published its second "Investors Dozen," a selection of stocks it thinks will do well in 1985. The Baltimore brokerage firm announced its first list in November, which has moved up 23 percent since Thanksgiving. The 1984 Legg Mason portfolio rose 29.88 percent, compared with 3.92 percent for the Standard & Poor's 500.

The new list contains two Richmond-based firms, CSX Corp., a transportation and natural resources company, and Owens and Minor Inc., distributor of medicines and hospital supplies.

The others are Beneficial Corp., Huffy Corp., IU International, Mercantile Bancorp, Moore Financial Group, Philips N.V., Public Service Electric and Gas (N.J.), Service Merchandise, Society Corp. and U.S. Steel's $2.25 cumulative convertible preferred. The average estimated 1985 price/earnings ratio for this list is 6.6, and the average yield is 5.4 percent.

Dominion Resources Inc. of Richmond permits its customers to buy stock through monthly contributions of as little as $10. They save brokerage fees and earn 8 percent interest until the stock is purchased once a year. The plan seems to be popular, with 30,020 customers participating in the 1984-'85 plan. Dominion says those who took part in the 1983-'84 plan earned an average rate of return of 32.2 percent on their investment.