By means of crossed immunoelectrophoresis with intermediate gel the occurrence of precipitating antibodies against Pseudomonas aeruginosa was studied in sera from 151 'normal' persons. A complex Pseudomonas aeruginosa antigen preparation and a corresponding rabbit antiserum were used as a reference system. Fifty-four per cent of the sera contained precipitating antibodies against Pseudomonas aeruginosa, and the prevalence of these antibodies increased with age. The specificity and titre of the precipitins were established in relation to the reference system, and the human Pseudomonas precipitins could in this way be referred to 2 Pseudomonas aeruginosa antigens that have been shown to cross-react with antigens from many other bacterial species. The precipitins were low-titred and could be absorbed with antigens from 7 other bacterial species. The implications of the results are discussed.