Adhesion of the periodontitis-associated bacteria Actinobacillus actinomycetemcomitans and Prevotella intermedia to monolayers of fibroblasts, HEp-2, KB and HeLa cells was quantified with radiolabeled bacteria. Bacterial adhesion was also examined microscopically with Giemsa-stained non-radioactive preparations. The degree of bacterial adherence was dependent on the growth phase of the bacteria. Strains at the exponential phase adhered to a greater extent than those at the stationary phase of growth. Both human and bovine lactoferrins competitively inhibited the adhesion of A. actinomycetemcomitans and P. intermedia to all tested cell monolayers. The inhibitory effect was dose-dependent in the concentration range 0.5-2500 micrograms/ml and not related to the bacterial growth phase. In the presence of lactoferrin, decreased association of bacteria with the cell monolayers was also found by microscopic examination of the preparations. The present findings indicate that lactoferrin may prevent the establishment of bacteria in periodontal tissues through adhesion-counteracting mechanisms in addition to its bacteriostatic and bactericidal properties.