The money keeps pouring into mutual funds: Investors pumped another $30 billion into stock, bond, and hybrid funds in May, according to the Investment Company Institute, the industry's main trade group. A figure that's less publicized than new sales but is equally important is redemptions. Even as one investor buys new shares in a fund, another may be cashing out. The difference, known as net sales or net flows, is the key to fund companies' profits.

"The industry doesn't focus very hard on the retention side," said Geoffrey Bobroff, a mutual fund consultant in E. Greenwich, R.I. "But revenues to advisers are often based on net flows. That's the important number. "

Over the past three years, Massachusetts Financial Services, a Boston- based fund company, has distinguished itself by steadily boosting its net sales. In almost every quarter, from April 1995 until April of this year, the company's net flows increased, according to data from Financial Research Corp., Boston. The company had net sales of $182 million in the second quarter of 1995 and surpassed the $1 billion mark in the same quarter a year later. By the first quarter of this year, Massachusetts Financial's net sales broke the $2 billion mark.

By contrast, Colonial Mutual Funds, also of Boston, has floundered. Its investors pulled more money out of the company's funds than they invested in them during every single quarter over the three-year period. The low point was the first quarter of 1997, during which net outflows were $298 million.

That kind of bleeding can stunt asset growth. For example, in the 12- month period ended April 30, Colonial's assets under management grew 7%. Massachusetts Financial, by contrast, saw its assets increase 55% over the same period, according to Financial Research.

The companies' differing fortunes have much to do with the stock market boom and investors' de-emphasis on bonds. Massachusetts Financial has thrived because its equity funds have performed so well. Its Massachusetts Investors Trust growth and income fund, for example, has outperformed 98% of the funds in its peer group over three years, according to Lipper Analytical Services. Massachusetts Financial, once known as a bond fund shop, has trumpeted that kind of performance to solidify its image as an equity fund manager at a time when investors are hungry for stock funds.

Today about 90% of the company's new sales are of stock funds and 10% are bond funds-a complete reversal from a decade ago.

Colonial, meanwhile, has remained saddled with the image of a bond fund company. But as it publicizes the performance of its equity funds and adds new stock funds to its selection, things seem to be turning around. In April, Colonial had $37 million in net sales.

"It takes time," Mr. Bobroff said, "especially with the perception on the street that you're an income shop."

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