In recent business cycle downturns, monetary policymakers worldwide have sought to stimulate their economies by conducting asset purchases. The U.S. Federal Reserve purchased both agency mortgage-backed securities (MBS) and Treasury (TSY) securities, which are generally thought to be comparable in credit quality and stimulative effects. This paper investigates the effect of such purchases on mortgage lending, commercial lending, and firm investment using micro-level data. We find that MBS and TSY purchases have asymmetric effects. In response to MBS purchases, banks that are active in the MBS market increase their mortgage origination market share, compared to other banks. At the same time, these banks reduce commercial lending. As a result, firms that borrow from these banks decrease investment. The effect of TSY purchases is either positive, as expected, or insignificant in most cases. Our results suggest different effects depending on the type of asset purchased, that MBS purchases cause distortionary effects across banks and firms, and that TSY purchases did not cause a large positive stimulus to the economy through the bank lending channel.