The effect of cefaclor on the steady state pharmacokinetics of theophylline was investigated in healthy young adults by comparing the pharmacokinetic parameters as found during a nine days course of theophylline alone and as obtained during co-medication with the antibiotic cefaclor. The pharmacokinetic parameters obtained on Day 9 during the two periods of drug treatment accounted for the following figures respectively: minimum plasma concentration 5.0 +/- 1.7 mg X 1(-1) (mean +/- SD) and 6.0 +/- 2.0 mg X 1(-1); maximum plasma concentration 9.7 +/- 2.6 mg X 1(-1) and 9.0 +/- 2.6 mg X 1(-1); time to peak 3.2 +/- 0.4 h and 3.9 +/- 0.8 h; AUC 132.54 +/- 41.73 mg X 1(-1) X h and 126.90 +/- 40.45 mg X 1(-1) X h; half-life of elimination 6.6 +/- 1.6 h and 7.1 +/- 1.2 h; clearance 0.059 +/- 0.011 1 X h-1 X kg-1 and 0.063 +/- 0.014 1 X h-1 X kg-1; volume of distribution 0.54 +/- 0.09 1 X kg-1 and 0.64 +/- 0.17 1 X kg-1. Only the values for cmax and tmax after cefaclor co-treatment were slightly, but significantly (p less than 0.05) different from the values found after administration of theophylline alone. As the volume of distribution and the clearance were not affected by cefaclor, it is concluded that both drugs can be given concomitantly without any dosage adjustment of theophylline.