Screening of normal plasma obtained from 172 blood donors from the Helsinki area and from 46 blood donors from the Moscow area was performed in order to reveal 'natural' antibodies to the common polysaccharide (rhamnan) and protein antigens of P. aeruginosa. Antibodies were detected by ELISA. Among blood donors from the Helsinki area high titres of antibodies to the protein antigens were detected in 42 active blood donors (24.4%) and very high titres in nine (5.3%) highly-active blood donors, whereas in the Moscow area in 15 (34.9%) and in one case (2.3%), respectively. Antibodies to the common polysaccharide antigen were determined in the Helsinki area in 23 active blood donors (13.4%) and in one (0.5%) highly active blood donor, whereas in the Moscow area in four active blood donors (8.6%). The plasma contained both polysaccharide and anti-protein antibodies. The level of antibodies to the polysaccharide antigen was lower than the level of antibodies to the protein antigens. There was no statistically significant difference between the corresponding values of blood donor groups from the Helsinki and Moscow areas.