Because many antibiotics are excreted into the breast milk, it can be difficult for a practitioner to choose an antibiotic for a lactating patient that will have minimal risks to her nursing infant. This article is the first of a three-part series discussing the use of anti-infective agents during lactation. The authors review general information regarding use and common side effects of several classes of antibiotics. They also summarize information, including documented milk concentrations, milk-to-plasma ratios, and other pharmacokinetic properties, in a table that can help practitioners choose antibiotics that may be considered safe for the lactating mother.