Shearson Lehman Hutton Inc. says it lost $15 million in the first quarter, attributing the $90 million decline from last year to sluggish Wall Street business activity and higher interest rates.

Shearson is the fifth investment firm to report a loss or lower earnings in the last week. In addition, Drexel Burnham Lambert Inc. said it is selling its retail brokerage unit, in part because of a business decline.Shearson said the loss for the quarter compared with a $75 million profit, or 75 cents a share, in the same 1988 period.

The firm said the decline reflects a $35.2 million pretax write-down of MCorp securities held by the firm, lower investment banking revenue and lower net interest income.

The decline came despite an increase in revenue to $3 billion from $2.5 billion a year ago.