Over five years 18 strains of Legionella pneumophila serogroup 6 were isolated in Amsterdam from the hot water supply in three hospitals and from one patient. Immunodiffusion and immunoblot procedures showed that these strains were identical. Profiles of isolated lipopolysaccharides from the 18 strains and the reference serogroup 6 strain were visualised in polyacrylamide gels stained with silver. Four strains from hospital A, isolated in 1982, 1984, and 1985 displayed similar lipopolysaccharide profiles which were different in relative mobility from those of hospitals B and C. Those from hospital B (12 strains isolated in 1983 and 1986) and C (one strain) were similar in relative mobility but different in colour. The strain from a patient with acquired immune deficiency syndrome (AIDS) in hospital A displayed a lipopolysaccharide profile characteristic of hospital A. These reproducible profiles were all different in relative mobility from the reference serogroup 6 strain. They can be used as a marker system in epidemiological surveys of serologically identical serogroup 6 strains. Lipopolysaccharide patterns from strains isolated throughout the years in the same hospital were similar. This suggests an outgrowth from organisms inhabiting the plumbing system rather than reseeding from the Amsterdam mains supply.