Initial immunological defense mechanisms to pathogen invasion rely on innate pathways of chemotaxis and phagocytosis, original to ancient phagocytes. Although chemotaxis has been well-studied in mammalian and model systems using purified chemoattractants in defined conditions, directed movement toward live bacteria has been more difficult to assess.Dictyostelium discoideumis a professional phagocyte that chemotaxes toward bacteria during growth-phase in a process to locate nutrient sources. UsingDictyosteliumas a model, we have developed a system that is able to quantify chemotaxis to very high sensitivity. Here,Dictyosteliumcan detect various chemoattractants at concentrations <1 nM. Given this exceedingly sensitive signal response,Dictyosteliumwill migrate directionally toward live gram positive and gram negative bacteria, in a highly quantifiable manner, and dependent upon bacterially-secreted chemoattractants. Additionally, we have developed a real-time, quantitative assay for phagocytosis of live gram positive and gram negative bacteria. To extend the analyses of endocytic functions, we further modified the system to quantify cellular uptakeviamacropinocytosis of smaller (<100 kDa) molecules. These various approaches provide novel means to dissect potential for identification of novel chemoattractants and mechanistic factors that are essential for chemotaxis, phagocytosis, and/or macropinocytosis and for more detailed understanding in host-pathogen interactive defenses.