Aspects of the interaction of certain mycoplasmas with macrophages and neutrophils in vivo and in vitro have been studied using two systems, one involving M. pulmonis in mice and the other involving M. bovis with bovine leucocytes. Studies with M. pulmonis indicated that the disappearance of viable organisms from the peritoneal cavity was not enhanced in SPF mice in which a peritoneal exudate rich in neutrophils had been induced. However, viable M. pulmonis organisms disappeared more rapidly from the peritoneal cavities with exudates containing increased numbers of macrophages. Experiments in vitro studied the opsonic effect of bovine IgG isotypes for bovine neutrophils and alveolar macrophages. Both IgG1 and IgG2 promoted killing of M. bovis by alveolar macrophages but IgG2 was more effective than IgG1 at promoting mycoplasma killing by neutrophils. Further studies in vitro indicated that certain bovine mycoplasma could inhibit killing of Escherichia coli by bovine neutrophils.