The adherence of 12 oral isolates and 4 type strains of Enterobacteriaceae (equally representing Enterobacter cloacae, Klebsiella pneumoniae, Escherichia coli and Citrobacter freundii) to HeLa cell monolayers following five different incubation conditions (sucrose, D-mannose, serum, MEM and Candida albicans GDH 1957) was investigated. Incubation with sucrose and D-mannose resulted in the greatest and least adherence, respectively. The presence of preadherent C. albicans GDH 1957 on the HeLa cells tended to enhance the adherence of certain strains of E. cloacae and C. freundii, but had no overall impact on Enterobacteriaceae adherence. While heterogeneity of behaviour existed between strains within species, E. cloacae was the most, and K. pneumoniae the least, adherent species irrespective of incubation conditions. Haemagglutination assays indicated the presence of mannose-resistant type 1 fimbriae associated with all Enterobacteriaceae. In clinical terms, the variations in adherence properties observed in vitro may contribute to an understanding of the different prevalence rates of oral Enterobacteriaceae reported in the literature.