The kinetics of elimination into milk of sodium penicillin G, procaine penicillin G, benzathine penicillin G, ampicillin and amoxycillin residues have been determined after intramuscular administration of eleven drugs chosen among those commercially available in France. These investigations will be used as a basis to estimate and harmonize the withdrawal times demanded for veterinary drugs. The quantitative analysis of residues was carried out by a cylinder plate microbiological method with Bacillus stearothermophilus as test organism. The threshold of detection is 0–001 unit (or μg)/ml of milk. The mean durations of elimination are four milkings for the association sodium penicillin G and procaine penicillin G, from seven to eight milkings for procaine penicillin G alone, from 19 to 33 milkings for benzathine penicillin G, and from three to five milkings for ampicillin and amoxycillin.