The concentrations of clarithromycin after a single oral dose of 500 mg were determined in serum and saliva of ten healthy adult volunteers. After 2 h, the levels were 4.04 +/- 1.14 (SD) mg/l in serum and 2.72 +/- 0.87 mg/l in saliva. After 7 h these values were 1.98 +/- 0.65 and 1.21 +/- 0.34 mg/l, respectively, and at the end of the dosing interval of 12 h, 0.95 +/- 0.38 and 0.73 +/- 0.35 mg/l, respectively. In comparison, amoxicillin (a 750-mg single dose) showed a much faster decline of serum levels and was virtually undetectable in saliva.