Serum from 154 normal persons was investigated for precipitating antibodies against P. aeruginosa by crossed immunoelectrophoresis, using a polyvalent P. aeruginosa standard antigen. Ninety-four per cent of the sera contained no demonstrable precipitins, and 6% contained precipitins against only one P. aeruginosa antigen. Thirty-four per cent of sera from 122 patients colonized with P. aeruginosa in various anatomical regions contained precipitins against 2 to 41 P. aeruginosa antigens. Serum from 14 patients harbouring mucoid strains of P. aeruginsa contained precipitants against these bacteria, and the average number of precipitins was significantly higher than the average number of precipitins in serum from patients harbouring non-mucoid strains. The largest number of precipitins was found in serum from adult patients chronically colonized with mucoid P. aeruginosa in the lower respiratory tract, and from adult patients with P. aeruginosa bacteremia originating from suppurative urinary tract infections caused by mucoid strains. Serum from patients with P. aeruginosa cystitis or from patients with P. aeruginosa colonization of the middle ear or with wound infection, contained only few or no precipitins.