In 23 women in childbed suffering from mastitis puerperalis the transport of oxacillin and ampicillin into the milk was investigated. Moreover the concentration of the antibiotics in the urine of babies nursed during therapy of their mothers with antibiotics was measured. After a regular taking (good compliance) the drug-concentrations in the serum and the milk of the mothers are higher than the minimal inhibiting concentrations necessary against the number of bacteria found in the milk. In the urine of suckling babies drug concentrations were differently high or undetectable as well. During the therapy of nursing mothers with antibiotics the milk should be misplaced.