Synthetic routes to a series of mono- and difluorinated 2-(4-amino-3-substituted-phenyl)benzothiazoles have been devised. Whereas mixtures of regioisomeric 5- and 7-fluoro-benzothiazoles were formed from the established Jacobsen cyclization of precursor 3-fluoro-thiobenzanilides, two modifications to this general process have allowed the synthesis of pure samples of these target compounds. Fluorinated 2-(4-aminophenyl)benzothiazoles were potently cytotoxic (GI(50) < 1 nM) in vitro in sensitive human breast MCF-7 (ER+) and MDA 468 (ER-) cell lines but inactive (GI(50) > 10 microM) against PC 3 prostate, nonmalignant HBL 100 breast, and HCT 116 colon cells. The biphasic dose-response relationship characteristically shown by the benzothiazole series against sensitive cell lines was exhibited by the 4- and 6-fluoro-benzothiazoles (10b,d) but not by the 5- and 7-fluoro-benzothiazoles (10h,i). The most potent broad spectrum agent in the NCI cell panel was 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (10h) which, unlike the 6-fluoro isomer (10d), produces no exportable metabolites in the presence of sensitive MCF-7 cells. Induction of cytochrome P450 CYP1A1, a crucial event in determining the antitumor specificity of this series of benzothiazoles, was not compromised. 10h is currently the focus of pharmaceutical and preclinical development.