This should help.
Under Mass. General Laws, ch.186, Â§15B, at the beginning of the tenancy, you may charge only:
rent for the first month;
rent for the last month, calculated at the same rate as the first month;
a security deposit equal to the first month's rent; and
the purchase and installation cost for a key and lock. This does not mean a key deposit. The law does not allow a key deposit.
The law also regulates how you can accept and hold these funds. For a security deposit, you must:
place it in an escrow account in a Massachusetts bank free from the reach of your creditors;
transfer the deposit to the new owner when you transfer the premises;
give the tenant a receipt showing the amount of the deposit, your name, the address of the premises, and the name of the bank and the account number in which the security deposit is being held;
give the tenant a statement of the present condition of the premises (he Greater Boston Real Estate Board has a form to comply with this requirement);
if the tenant submits to you a separate list of damages, you must return a copy of the tenant's list to the tenant within fifteen days of receiving it, with either your signed agreement with the list or a clear statement of your disagreement attached;
pay the tenant 5% interest per year on the deposit, or the amount of interest you receive from the bank each year if that is less;
keep careful records on the security deposit and make them available to the tenant at your office during normal business hours.
You must return the deposit within 30 days after the tenant vacates, if a tenant at will, or the 30 days after the expiration of the lease. You may only deduct unpaid rent which has not been validly withheld or deducted, taxes due under an escalator clause, and the cost of damage the tenant has done to the premises (this does not include normal wear and tear). If you deduct for damages, you must follow the procedure in the statute exactly.
Hope that helps,