This study examined the disposition of olsalazine and its metabolites into breast milk after the ingestion of a single dose of 500 mg olsalazine. Blood and serum samples were obtained for 48 hours after the ingestion of 500 mg olsalazine in a 39-year-old lactating woman. Blood samples were obtained at .0, .5, 1, 2, 4,6, 24.5, 26, and 48 hours. Maternal milk samples were obtained at .0, .5, 2, 4, 6, 14, 24, 28, 36, and 48 hours. Olsalazine and olsalazine-S underwent high-pressure liquid chromatography analysis, and 5-ASA and Ac 5-ASA underwent fluorometric detection. Acetylated-5-ASA achieved concentrations of .8, .86, and 1.24 mumol/L in breast milk at 10, 14, and 24 hours, respectively. Olsalazine, olsalazine-S, and 5-ASA were undetectable in the breast milk for 48 hours after drug administration. Clinically significant drug exposure in the breast-fed infant is unlikely after a maternal single dose of olsalazine. Idiosyncratic hypersensitivity, however, remains a possibility even if the infant is exposed to only minute quantities.