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2-(4-Aminophenyl)benzothiazole molecules substituted in the 3 position of the phenyl ring with a halogen atom or methyl moiety comprise a group of compounds that potently inhibit specific human ovarian carcinoma cell lines. GI50 values fall within the nM range. Inhibition is highly selective -- whereas the GI50 value in IGROV1 cells consistently lies at < 10 nM, SK-OV-3 presents GI50 values > 10 microM. Biphasic dose-response relationships were observed in sensitive cell lines after 48-h drug exposure. COMPARE analyses revealed the very similar profiles of anti-tumour activity of 3-substituted benzothiazoles and 5-(4-dimethylaminophenylazo)quinoline, with Pearson correlation coefficients > 0.65. Anti-tumour activity extended to preliminary in vivo tests. The growth of OVCAR-3 cells in polyvinylidene fluoride (PVDF) hollow fibres implanted in the peritoneal cavity of mice was inhibited by more than 50% after intraperitoneal (i.p.) administration of 2-(4-amino-3-methylphenyl)benzothiazole (10 mg kg(-1)), 2-(4-amino-3-chlorophenyl)benzothiazole (100 mg kg(-1)) or 2-(4-amino-3-bromophenyl)benzothiazole (150 mg kg(-1)). The growth of OVCAR-3 tumours in subcutaneously (s.c.) implanted hollow fibres was retarded by more than 50% after treatment with 2-(4-amino-3-methylphenyl)benzothiazole (6.7 and 10 mg kg(-1)). In addition, the growth of s.c. OVCAR-3 xenografts was delayed after exposure to DF 203. However, the relationship between drug concentration and growth inhibition was inverse.