The biochemical composition of "unstimulated" whole saliva was determined in healthy adult subjects. Based on their relative concentration, salivary analytes could be classified into three arbitrary categories: concentration lower than in serum (saliva/serum ratio < 0.5; 12 analytes), similar to serum (ratio = 0.5–1.5; five analytes), and higher than in serum (ratio > 1.5; five analytes). Consistent with local production, an elevated lactate dehydrogenase (LDH) activity in the saliva was associated with a non-serum like LDH isoenzyme pattern: LDH5 >> LDH4 > LDH3 >> LDH2 > LDH1. Compared with serum, the concentrations of hydrogen (as reflected in the pH), potassium and inorganic phosphorus were much higher (saliva/serum ratio ≥ 3), whereas that of sodium, total magnesium, chloride, and total carbon dioxide were lower (saliva/serum ratio ≤ 0.3). The concentration of ionized calcium was similar in saliva and serum (saliva/serum ratio = 0.8), while ionized magnesium was unmeasurable in saliva. The salivary ionized calcium fraction was higher (0.76) than previously suggested (0.51). The difference between the main salivary cations (potassium, sodium), and anions (phosphate, chloride) was similar to serum (anion gap: 4 vs. 11 meq/l). Highly significant (p ≤ 0.012) correlations occured among salivary pH, dihydrophosphate, total calcium, and potassium. Our data suggest that calcium, potassium, chloride and phosphates are the major salivary complex-forming ions. The major compositional differences between serum and saliva show that saliva is not a passive "ultrafiltrate" of serum and salivary constituents may play a distinct physiological role.