The isolation of Actinomyces naeslundii from sound, exposed root surfaces (n = 56) and soft and leathery root carious lesions (n = 71) was investigated. Root carious lesions were sampled after the removal of overlying plaque. Supragingival plaque or carious dentine was sampled using a sterile excavator, the samples were disaggregated and cultured on both selective and non-selective media. A. naeslundii isolates were identified to the genospecies using specific antisera. Significantly greater numbers and proportions of A. naeslundii genospecies 2 than A. naeslundii genospecies 1 were isolated from all sites sampled. There was no significant difference between the numbers and proportions of the two genospecies isolated from leathery and soft lesions. The relationship between the presence of A. naeslundii genospecies and aciduric and acidogenic organisms was investigated. Those sound exposed root surfaces from which A. naeslundii genospecies 1 and/or 2 were isolated yielded significantly lower numbers of lactobacilli and yeasts than the surfaces from which A. naeslundii were not isolated. This difference was also found in leathery lesions but not soft root carious lesions. The microflora of soft root carious lesions was found to comprise primarily gram-positive pleomorphic rods which formed 70+/-7.8% of the flora, while in plaque from exposed root surfaces and in infected dentine from leathery lesions the gram-positive pleomorphic rods represented only 35% of the flora.