The composition of pooled resting plaque fluid was determined in four groups of college-age students (18-22 years), each composed of 50 individuals, who abstained from oral hygiene for 36 hours and did not eat or drink for at least one hour prior to plaque collection. Plaque samples from each group were pooled under mineral oil in small centrifuge tubes and centrifuged at 37,000 g for one hour at 4 degrees C. Supernatants were then combined under mineral oil and centrifuged at 5000 g (4 degrees C) for 15 minutes. In general, the inorganic composition of plaque fluid in the four groups was quite similar and in agreement with values reported by other investigators, but quite different from those of saliva or serum. The mean composition was: Ca, 7.07 +/- 0.51 mmol/L; P, 23.2 +/- 5.3 mmol/L; Na, 18.6 +/- 2 mmol/L; K, 85.1 +/- 5.3 mmol/L; Mg, 3.9 mmol/L; Cl, 42.8 +/- 9 mmol/L; F, approximately 0.004 mmol/L; pH, 5.69 (5.63-6.01). Acetate, propionate, succinate, butyrate, lactate, and formate were determined in two samples analyzed, with acetate and propionate being the predominant acids found. It was also demonstrated, through the titration of one of the plaque fluid samples, that the observed buffer capacity in plaque fluid was mostly related to its organic acid composition. It was noted, however, that when the initial pH in plaque fluid exceeded 6.5, phosphate contributed significantly to the buffer capacity. The contribution of other soluble species (proteins, peptides, amino acids) to the observed buffering in plaque fluid appeared to be small.(ABSTRACT TRUNCATED AT 250 WORDS)