All biomaterials, including biologic scaffolds composed of extracellular matrix (ECM), elicit a host immune response when implanted. The type and intensity of this response depends in part upon the thoroughness of decellularization and removal of cell debris from the source tissue. Proinflammatory responses have been associated with negative downstream remodeling events including scar tissue formation, encapsulation, and seroma formation. The relative effects of specific cellular components upon the inflammatory response are not known. The objective of the present study was to determine the effect of different cell remnants that may be present in ECM scaffold materials upon the host innate immune response, both in vitro and in vivo. Collagen scaffolds were supplemented with one of three different concentrations of DNA, mitochondria, or cell membranes. Murine macrophages were exposed to the various supplemented scaffolds and the effect upon macrophage phenotype was evaluated. In vivo studies were performed using an abdominal wall defect model in the rat to evaluate the effect of the scaffolds upon the macrophage response. Murine macrophages exposed in vitro to scaffolds supplemented with DNA, mitochondria, and cell membranes showed increased expression of proinflammatory M1 marker iNOS and no expression of the proremodeling M2 marker Fizz1 regardless of supplementation concentration. A dose-dependent response was observed in the rat model for collagen scaffolds supplemented with cell remnants. DNA, mitochondria, and cell membrane remnants in collagen scaffolds promote a proinflammatory M1 macrophage phenotype in vivo and in vitro. These results reinforce the importance of a thorough decellularization process for ECM biologic scaffold materials. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2109-2118, 2017.