We measured adenosine deaminase (ADA) activity in a guinea pig model of Legionella pneumophila infection. Female Hartley guinea pigs were inoculated intraperitoneally with one-quarter of the LD50 dose of L. pneumophila Philadelphia-1 strain. Control groups were inoculated with clinical isolates of Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae or Klebsiella pneumoniae. Each group consisted of 5 animals. ADA activity in plasma was assayed calorimetrically before and at various intervals after infection by measuring the amount of ammonia produced after adenosine was added to plasma samples. ADA activity before inoculation was 25.6 +/- 6.0 IU/1, it reached 174.4 +/- 60.0 IU/1 on day 3 after inoculation of L. pneumophila. ADA activity returned to normal levels on day 14. ADA activity did not increase significantly in guinea pigs infected with the other types of bacteria. These findings suggest that measurement of plasma ADA activity may be useful for the diagnosis of Legionella infection.