- Abstract One of the more substantial hurdles to be overcome in realizing the exciting potential of siRNA molecules as therapeutic agents for a wide range of diseases is the intact delivery of the active molecule into its target cell. Here, we present a platform for in vitro and in vivo delivery and intracellular release of siRNA in peritoneal macrophages (Mϕs). The delivery platform is based on the encapsulation of siRNA in biodegradable poly( d,l -lactide) (PLA) microspheres, which are targeted to Mϕs by the simple principle of size exclusion. Proof of concept was achieved using siRNAs targeting TNFα and CD86 in macrophages. We show that the release of the siRNA in peritoneal-derived macrophages in vitro occurs intracellularly, and is abrogated by cytochalasin B, a phagocytosis inhibitor. Silencing in these cells is potent and lasts for at least one week. In vivo , we prove that siRNA encapsulated in biodegradable PLA microspheres can be delivered to peritoneal-residing Mϕs and can induce potent silencing of TNFα secretion for at least one week. The PLA microspheres hold great potential for in vivo use, due to their biocompatibility and degradability, and can potentially be used for in vivo immunomodulation of Mϕs for treatment of autoimmune and chronic inflammatory conditions.