The administration of intravenous fluids or drugs may allow particulate and microbiologic contamination to enter the bloodstream. Studies have implicated injected particles as a cause of postinfusion phlebitis. The process of manufacturing the infusion product will influence the type and quantify of injected particles. Particles greater than 8 microns are filtered by the lung, and depending on their reactivity, can cause pulmonary granulomas. Particles below 8 microns are initially filtered, cleared by phagocytosis, and ultimately migrate to the liver and spleen. Both nonvisible particles and microbial contaminants of fluids and drugs can be effectively removed by 0.45- or 0.22-micron in-line final filtration. When filters are employed, problems can occur, including air locking, clogging, and drug-binding. A knowledge of why and to what extent these problems can occur will allow in-line filters to be used effectively by health care practitioners.