Little detailed knowledge exists about the composition of the human root flora. Therefore, the predominant cultivable flora (PCF) was determined for samples of: (1) plaque from sound root surfaces (SRS) in eight subjects without root caries, (2) plaque from incipient root lesions (IRL) in eight subjects with root caries, and (3) carious material from advanced root lesions (ARL) in nine other subjects with root caries. Generally, one root surface was sampled per subject, and organisms-358, 512, and 389 for SRS, IRL, and ARL, respectively-were identified by standard methods. It was found that: (1) streptococci, actinomyces, and veillonellae constituted 84.2, 57.8, and 65.7% of the PCF of SRS, IRL, and ARL samples, respectively; (2) a wide variety of other Gram-positive cocci and Gram-positive and -negative rods was also present; (3) the PCF of many samples was often dominated by few organisms, the identity of which differed from sample to sample; (4) a negative and no association with root caries existed for the PCF levels of the non-mutans streptococci (non-MS) and the actinomyces; and (5) mutans streptococci (MS) and lactobacilli (L) were not always present among the PCF of IRL and ARL, respectively, and non-MS were isolated from the PCF of most lesions and actinomyces from all lesions. Our findings illustrate the complexity of the root surface flora and suggest that root caries development involves organisms other than MS and L.