Mass. Gen. Laws ch. 244, § 28: Interest

Section 28. If the court finds that the mortgagee has not unreasonably neglected or refused to render a true account of rents and profits of the land mortgaged, it may award him the balance found due on the mortgage, with interest thereon at a rate of not more than twelve per cent a year from the expiration of three years after the entry to the date of the judgment.