Eight volunteers were each given 300 mg of erythromycin lactobionate by i.v. infusion over 15 min in the presence and absence of chronic dosing with slow-release theophylline. Pharmacokinetic profiles were obtained for theophylline in the presence and absence of erythromycin and for erythromycin in the presence and absence of theophylline. A very small, clinically unimportant, but statistically significant increase occurred in mean (+/- S.E.M.) serum theophylline concentration from 4.9 +/- 0.3 mg/l to 5.2 +/- 0.3 mg/l in the presence of erythromycin (P = less than 0.01). The theophylline pharmacokinetic parameters did not change significantly. The only changes in erythromycin pharmacokinetics were an increase in the renal excretion (0-12 h) from 5.5 +/- 4.0 mg to 11.2 +/- 6.0 mg (P less than 0.03) and an increase in renal clearance (0-2 h) from 9.0 +/- 6.0 ml/min to 21.6 +/- 15 ml/min (P less than 0.05) in the presence of theophylline.