A capillary gas chromatographic method is described for determining residues of beta-lactam antibiotic residues in milk, with specificity for benzylpenicillin (penicillin G), phenoxymethylpenicillin, methicillin, oxacillin, cloxacillin, dicloxacillin, and nafcillin. Residues are extracted from milk with acetonitrile. Samples are cleaned up by partitioning between aqueous and organic phases at different pH values. The penicillin residues are methylated with diazomethane to render them amenable to determination by gas chromatography on a methyl silicone fused silica column. Samples are introduced by split/splitless injection using a programmed temperature vaporization injector and are detected by nitrogen-selective thermionic detection. Internal standardization is used for quantitation. The limits of detection for all penicillins are well below 1 microgram/kg. Recoveries of spiked samples at 3 and 10 micrograms/kg are in the range of 42-85% (coefficients of variation 2-5%) and 41-92% (coefficients of variation 3-7%), respectively.