A continuous survey of the serological response in 59 Pseudomonas aeruginosa infections of 30 patients treated in a respiratory intensive care unit revealed that anti-LPS antibody production paralleled the intensity of the infection from the early onset. In lethal cases the immunological response was depressed from the start of the P. aeruginosa complications, probably as a consequence of a too intensive antigenic stimulus exerted by a massive infection. Septic shock was always accompanied by a marked fall in antibody titres. The loss was more expressed in the most effective IgG class antibodies.