The pharmacokinetic pattern of erythromycin propionate-N-acetylcysteinate (EPAC) (erythromycin stinoprate I.N.N.), a new derivative, was studied on 12 healthy volunteers after single and multiple oral treatments. Microbiological and/or HPLC analytical methods were used to titer either erythromycin as base, propionate and total or N-acetylcysteine (NAC). In the acute experiment, a comparative evaluation was performed with erythromycin stearate (ES) and with N-acetylcysteine, according to a randomized-multi-crossover design. EPAC showed a better bioavailability than ES with longer-lasting serum levels of active antibiotic. NAC concentrations in the serum after EPAC were practically identical to those found after an oral administration of NAC alone. The multiple treatment study, performed in the same 12 volunteers with only EPAC, indicated that the pharmacokinetic pattern is somewhat different from that observed after a single dose, since higher concentrations were present at the steady state conditions.