About 2,500 workers in Illinois and 1,500 in Michigan, will lose their jobs over the next two year according to reports from Reuters.
The US bank wants to save as much as $800 million by 2009 and cuts are seen as the best way to do it, but the bank expects to employ about 8,000 people in Illinois and 2,500 in Michigan later on.
"We're buying LaSalle with the goal of growing the combined organization. The layoffs are a necessary first step toward meeting that long-term goal," Scott Silvestri, a spokesman, told the news agency.
The deal has already received approval from the Fed and the Charlotte, North Carolina based bank is expected to close the deal in early October.
LaSalle has assets estimated at $160 billion, along with over 400 branches and 1.4 million customers.