The elimination into bovine milk ofß‐lactam antibiotic residues (procaine penicillin G, cloxacillin, ampicillin, oxacillin, cephalexin) following intramammary administration of 10 preparations marketed in France was studied. The quantitative analysis of residues was carried out by a microbiological agar diffusion method usingBacillus stearothermophilus.Sensitivity ranged from 0.001 I.U./ml for procaine penicillin G and 0.001μg/ml for ampicillin to 0.02μg/ml for cephalexin. The mean periods of elimination on which withholding times are based were between four and seven milkings according to the drugs administered.