In a first experiment, the relative virulence for pigeons of 5 strains of S. bovis was assessed by experimental inoculations. Two S. bovis serotype 1 strains, one serotype 2 strain and two serotype 3 strains were examined. One of the serotype 1 strains and the serotype 2 strain were isolated from pigeons that died from septicaemia. The other strains were isolated from cloaca samples of healthy pigeons. For each strain, 10-20 pigeons were intravenously inoculated with 1 x 10(9) CFU. Morbidity after infection with the serotype 1 and 2 strains varied between 75% and 90%. Disease signs included inability to fly, lameness, emaciation, production of slimy, green droppings, polyuria and sudden death. In groups of pigeons inoculated with the serotype 3 strains, morbidity was 0% and 6%, respectively. Results demonstrate that serotype 3 strains are less virulent for pigeons than serotype 1 and 2 strains. In a second experiment, bacteriological and histological examinations were performed on organs of pigeons serially killed between 1 and 10 days after experimental inoculation with an S. bovis serotype 3 strain of low virulence. Results were compared with results of studies carried out with a highly virulent serotype 1 strain. Notwithstanding bacterial spread and replication in various organs of inoculated pigeons, clinical disease was not observed and histological lesions were scarce and of limited extent.