BR96 sFv-PE40 is a single-chain immunotoxin fusion protein targeted to the Ley Ag, which is expressed in many different human carcinomas as well as in normal gastrointestinal epithelium of humans and certain animals, including athymic rats but not mice. In vitro binding analysis determined that BR96 sFv-PE40 was similar in affinity to BR96 Fab. BR96 sFv-PE40 internalizes rapidly, similar to BR96 IgG. H3396 cells, derived from metastatic human breast carcinoma, have been established as tumor xenografts in estradiol-supplemented athymic mice and rats. H3396 tumor xenografts established in athymic mice (up to 350 mm3) and rats (up to 100 mm3) completely regressed after i.v. administration of BR96 sFv-PE40, given as 0.625 mg/kg (1.975 mg/m2) every 4th day for a total of five doses (mice) or 0.25 mg/kg (1.475 mg/m2) every 4th day for a total of four doses (rats). The tumors remained regressed for the duration of the study (> 85 days post-implant), which represents > 10 doubling times, indicating that the animals were cured. There was no toxicity in rats receiving a curative dose of 0.25 mg/kg, although liver and lung toxicity could be detected at a 16 times higher dose, 4 mg/kg or 23.6 mg/m2. We conclude, therefore, that BR96 sFv-PE40 can cure tumor xenografts at well tolerated doses and also in the presence of Ley expression in normal tissues.