For its survival, the host depends on an elaborate internal defense system to protect against persistent invasion by pathogenic microorganisms. This system is composed primarily of cellular and humoral components of the circulatory system and includes the "professional" phagocytes (polymorphonuclear leukocytes, monocytes, and tissue macrophages), antibody, and complement. The humoral component is often able to deal with invaders singlehandedly, as by antibody-independent or antibody-dependent complement lysis. However, the cellular and humoral components must cooperate to destroy better-equipped pathogens by the process of phagocytosis. Hundreds of studies on phagocytosis of one or another microorganism have been published. Experimental conditions in these studies have varied widely. Investigators have employed a variety of white cell types (polymorphonuclear leukocytes, monocytes, or macrophages) from different anatomical regions (usually blood, peritoneum, or alveoli) of different animals (often human, rabbit, guinea pig, or mouse). These white cells have been obtained in their resident state or after elicitation