A series of 3-(2,4-dichloro-5-fluorophenyl)-6-(substituted phenyl)-1,2,4-triazolo[3,4-b]-1,3,4-thiadiazines (4) (Fig. 1) have been synthesized by the cyclization of 3-(2,4-dichloro-5-fluorophenyl)-1,2,4-triazol-5-thiol (3) with substituted phenacyl bromides. All the newly synthesized compounds were confirmed by IR, (1)H NMR and mass spectral studies. Among the compounds tested for their antitumor activity three compounds exhibited in vitro antitumor activity with moderate to excellent growth inhibition against a panel of sixty cancer cell lines of leukemia, non-small cell lung cancer, melanoma, ovarian cancer, prostate and breast cancer. The compound 4d showed promising antiproliferative activity with GI(50) values in the range of 1.06-25.4 microM.