Thirty antibiotics, including penicillins, cephems, chloramphenicol, tetracyclines, macrolides, and aminoglycosides, were given p.o., i.m. or i.v. once to 70 puerperal women to compare milk drug levels with serum levels over a 6-h period. Antibiotic transfer into the milk was small. No drugs except for a few (chloramphenicol, tetracyclines, macrolides) reached the level of 1 mcg/ml or greater. Penicillins and cephams detected in trace amounts in the milk are likely to have little effect on infants.