Various techniques have been utilized for measurement of antibodies against Pseudomonas aeruginosa. In general, the results of such measurements tend to parallel each other, but the principal functional antibodies against Pseudomonas appear to be opsonins of the IgG and IgM immunoglobulin classes. These antibodies are potentiated by the complement system, activated either through the “classical” or “alternate” pathways. Susceptibility to fatal infection with Pseudomonas is associated with antibody deficiency, but this defect is usually correlated with other data regarding the state of the host, such as underlying disease, immunosuppressive therapy, and functional leukopenia. Additional information is needed on the relative protective role of IgG and IgM antibodies, the affinity of such antibodies for viable bacteria, and the role of antibodies against nonlipopoly-saccharide antigens, such as protein components of the cell wall and Pseudomonas exotoxins.