Boston Co.'s institutional money management division is rebounding this  summer, scoring points with clients after losing many substantial accounts   last year.   
In July, Boston Company Asset Management Inc., a subsidiary of Mellon  Bank Corp., attracted $116 million worth of new client assets, bringing the   total assets under management to $15.5 billion. But that's still about half   what the company was managing in spring 1995.     
  
The institutional division - one of 10 investment subsidiaries of Mellon  that manage a total of $237 billion - lost 40% of its business last year   because of an executive exodus.   
The firm has added 23 clients since the beginning of this year, and new  executives say they will accelerate that pace. 
  
"People have gotten over the events of last year. They understand that  Boston Company Asset Management is here to stay," said Francis D. Antin,   its chief operating officer.   
In April 1995, the division suffered a huge blow when 20 employees led  by former chief investment officer Desmond J. Heathwood walked away to set   up a competing firm - Boston Partners - after Mellon bought Boston Co. At   the end of June 1995, Boston Company Asset Management was holding onto $15   billion in client money, down from $26 billion at the time of the   defections.         
When Mr. Antin, an experienced marketing executive from State Street  Bank and Trust Co., stepped in last November, assets were down to $13.5   billion.   
  
Mr. Antin has worked with chief investment officer Alexander "Toby" Webb  3d, a former president and chief investment officer of Fidelity Management   Trust Co. who replaced Mr. Heathwood last summer, to convince clients and   prospects of the firm's abilities going forward.     
The largest new account was opened by the City of Boston Retirement  System, which invested $35 million in a small cap equity strategy. Another   shot in the arm came from an undisclosed insurance company that opened a   $25 million account.     
In core equity investments, separate accounts were opened by the  Jacksonville, Fla.-based Jesse duPont Foundation and Kansas City, Mo.-based   American Marketing Industries Inc. for $18 million and $11 million,   respectively.     
Maryknoll Charitable Trust, a foundation in New York  State, added $20 million to a balanced account. Two new clients, the   Massachusetts city of Worcester and town of North Attleboro, opened smaller   accounts for international equity investments that total $7 million.