Most investigations of mechanisms accounting for intergeneric coaggregation have emphasized stereospecific rather than nonspecific interactions. The purpose of this investigation was to determine the relative importance of lectin-carbohydrate and nonspecific hydrophobic and ionic interactions, using a model based on strains with one of the most well understood specific coaggregation mechanisms, the lactose-reversible coaggregation of Actinomyces naeslundii and Streptococcus oralis. The kinetics of coadhesion and desorption of coadherent bacteria were studied using S. oralis 34 bound to hexadecane droplets as an affinity support for the adhesion of A. naeslundii WVU 398A. Light, confocal microscopy and transmission electron microscopy confirmed that A. naeslundii cells adhered only to the S. oralis cells, not to exposed hexadecane between the streptococci. Coadhesion was inhibited by lactose concentrations as low as 2.0 mM. The rate of coadhesion was halved at 60 mM lactose. The hydrophobicity inhibitors bovine serum albumin and defatted bovine serum albumin and the salts LiCl and KCl failed to inhibit coadhesion in the hexadecane assay, and bovine serum albumin also failed to inhibit coaggregation in a bacterial aggregation assay on glass slides. High concentrations of the salts achieved a 50% rate decrease in A. naeslundii adhesion to the S. oralis-coated droplets only when they were combined with > 20 mM lactose. Sodium dodecyl sulfate (SDS) and Tween 20 inhibition was tested by the slide coaggregation assay because they tended to emulsify the droplets; SDS was inhibitory. Lactose selectively desorbed A. naeslundii from S. oralis-coated droplets at low concentrations equivalent to those that inhibited coadhesion. Neither LiCl nor KCl desorbed A. naeslundii from the droplets, even at 500 mM. At low concentrations, SDS but not Tween 20 eluted both A. naeslundii and S. oralis from the droplets. Although the SDS results might suggest a degree of cooperative charge interactions, the results support the hypothesis that stereospecific, beta-galactoside-sensitive interactions have a much greater impact than nonspecific interactions on the coadhesion of A. naeslundii and S. oralis.