This study examined the individual and combined effects of the selective agents normally present in Yersinia-selective agar (i.e. cefsulodin, irgasan and novobiocin) on the growth kinetics of plasmid-bearing (P+) and plasmid-cured (P-) Yersinia enterocolitica serotype O:3 at 25 and 37 degrees C. Growth studies were carried out in pure culture, and the data obtained were subjected to linear regression analysis to determine lag phase duration(s) and growth rates of the examined strains. In general, the presence of selective agents increased the duration of the lag phase at 37 degrees C, with longer lag phases noted in all cases in which two or more selective agents were present. Growth rates in CIN broth base (CIN NA) and CIN NA plus commercial supplement (SR 109) (CIN) were faster at 37 than 25 degrees C, but in some cultures of incomplete CIN NA broth with less than three supplements added, growth tended to be faster at 25 than 37 degrees C. Generally, plasmid-bearing strains grew slower than plasmid-cured strains in most media at 37 degrees C due to virulence plasmid expression retarding growth. In some instances at 37 degrees C, it was observed that the growth rates of both plasmid-bearing and plasmid-cured strains were comparable, indicating the influence of added selective agent/s negating any effects associated with virulence plasmid expression. The effects of selective agents, incubation temperature and virulence plasmid carriage on the growth kinetics of Y. enterocolitica are discussed.