Various factors affecting the aggregation of Actinomyces naeslundii strain 12104 were studied. When the pH of glucose-supplemented growth medium fell below 5.5, the cells aggregated and formed microbial masses which tenaciously adhered to the culture vessels. When the organism was cultured in the same medium in the absence of glucose, maximum growth was reduced and the final culture pH values remained above 6.5, but the cells were more dispersed and nonadherent. Adjusting the final pH of these cultures to below 5.5 with HCl caused the cells to aggregate. Cells from unsupplemented cultures with final pH values of 6.7 were washed by centrifugation, dispersed by vigorous shaking, and suspended in buffer at pH values ranging from 4.5 to 8.0. Aggregation (expressed as the percent reduction of optical density at 520 nm after incubation at 37 degrees C) occurred rapidly at pH values below 6.0 but did not readily occur at higher pH values. Aggregation of strain 12104 in washed cell suspensions was induced by low pH and influenced by cell concentration and ionic strength of the environment. Low pH values also induced aggregation in washed cell suspensions of Streptococcus mutans, Streptococcus sanguis, Streptococcus salivarius, and Actinomyces viscosus.