Chemotaxis of rabbit mononuclear cells was studied by the micropore-filter technique. Mononuclear cells obtained from mineral oil-induced peritoneal exudates respond chemotactically to rabbit serum treated with immune complexes or with streptokinase and plasminogen, to soluble factors produced by bacteria, and to lysates obtained from rabbit neutrophils. The first chemotactic factor requires heat-labile factors in serum for its generation, but, once formed, the chemotactic factor is relatively heat stable. This factor has been compared with the complement-associated factor in serum, C'(5, 6, 7)a, that is chemotactic for neutrophils. The ability to generate the mononuclear cell chemotactic factor in serum that has been treated with potassium thiocyanate suggests that complement is not required. The position of the chemotactic factor in preparative electrophoresis and density-gradient ultracentrifugation indicates that on the basis of physical-chemical criteria, this factor is not C'(5, 6, 7)a. The mononuclear cell chemotactic factor present in lysates of neutrophils sediments slowly in the ultracentrifuge and may be related, at least in part, to cationic peptides of lysosomal granules. A study in the time course of the chemotactic response of mononuclear cells reveals that the response begins to level off after 4 or 5 hr. This is in sharp contrast to the time course for the chemotactic response of neutrophils, in which the reaction is complete within 1.5 hr. Rabbit mononuclear cells obtained from a starch-induced peritoneal exudate respond to serum treated with plasminogen and streptokinase and to the soluble factor produced by bacteria, but no chemotactic response is elicited to serum treated with immune complexes. This indicates a functional difference between two populations of mononuclear cells. Rabbit alveolar macrophages respond poorly to all agents tested, although a weak chemotactic response to bacterial factors was found. The requirement of a serine esterase in the mononuclear cell for the cell to respond chemotactically was defined by the use of organophosphorus inhibitors. Pretreatment of mononuclear cells with several series of phosphonates renders them unresponsive in the chemotactic system. The effect is similar for mononuclear cells responding chemotactically to activated serum and to the bacterial chemotactic factor. Several points of contrast with inhibition profiles obtained in the chemotactic system of the neutrophil suggest that, while the mononuclear cell requires a serine esterase for chemotactic responsiveness, this enzyme is different from the one previously defined in the neutrophil.