The presence of amoebae and Legionella pneumophila in ground-water, drinking water supplies and whirlpools was investigated. Volumes of 10 to 1,000 ml were concentrated by membrane filtration. L. pneumophila was detected on buffered charcoal yeast extract (BCYE) agar, and amoebae by inverting filters on nonnutrient agar plates seeded with Escherichia coli that were incubated at 37 C for up to 12 days. In 65% of the samples positive for L. pneumophila amoebae were also detected. L. pneumophila and amoebae were detected together in 38% of warm drinking water samples. The highest isolation temperature for amoebae was 57 C, but fewer amoebae were detected above than below 50 C. In cold drinking water, amoebae were found in 88% of samples. The presence of L. pneumophila and amoebae in whirlpool waters (42%) presents a risk for man. Fresh environmental isolates of an Acanthamoeba species and L. pneumophila serogroup 4 were used for laboratory experiments. The amoebae supported intracellular multiplication of L. pneumophila in Chang's medium and autoclaved tap water, as shown by colony-forming unit (CFU) counts, direct fluorescent antibody test and Gimenez staining. Results confirmed that interaction between L. pneumophila and amoebae could occur in nature, and that the latter could act as hosts for legionellae and support their growth.