The capacity of national reference laboratories of the European Union member states to correctly serotype Salmonella strains was assessed in four collaborative studies on serotyping in the period 1995-1999. Participants were asked to identify 20 strains in studies I, II and III and 16 strains in study IV, using the typing method routinely performed in their laboratory. In the first study, the strains to be identified belonged to Salmonella enterica subsp. enterica, salamae or houtenae, while in the other studies only strains belonging to Salmonella enterica subsp. enterica were included. Significant differences between laboratories and between studies were found. Differences were related to the frequency of actual occurrence of the study strains in the area served by the laboratory and the number of antisera available in the laboratory.