The passage of two antibiotics, cephradine and epicillin, into the milk of 12 lactating women and the amniotic fluid of 48 pregnant women was investigated. All women were given a 500-mg capsule every six hours for at least two days prior to our taking multiple biologic samples from them. Constant levels of both antibiotics were reached in the milk of lactating women as well as in the amniotic fluid of mid-term and full-term pregnant women during the sample period. The ratio of drug concentrations in serum to that in milk was about 5.0 for each antibiotic. In the midtrimester women, the ratio of concentrations in serum to amniotic fluid of both antibiotics was approximately 1.0, suggesting the development of an equilibrium between these two compartments. This ratio was 0.2 for both drugs in the full-term women, demonstrating that the antibiotics concentrated in the amniotic fluid compartment.