New derivatives of imidazothiazole and imidazobenzothiazole were tested in vitro for their potential antiproliferative activity. Four imidazobenzothiazole derivatives exhibited a cytotoxic activity against two leukemic cell lines, compound I being the most effective. Cell cycle kinetics studies showed that this drug delays the progression of cells from G1 to S and G2 M phases. An inhibitory effect on DNA and RNA synthesis was also observed. The antiproliferative effect of this compound, analogue of immunosuppressive agents, suggested that it could be of interest for a therapeutic use and for the synthesis of new derivatives.