Several substituted 4-aryloxy- and 4-arylsulfanyl-phenyl-2-aminothiazoles were synthesized and evaluated for cytotoxic activity against estrogen-positive, estrogen-negative, and adriamycin-resistant human breast cancer cell lines. 4-[4'-(3,4-Dichlorophenoxy)-phenyl]-thiazol-2-yl ammonium iodide demonstrated potent activity against both estrogen-positive and negative breast cancer cell lines with low micromolar (microM) GI(50) values. In addition, we have identified several 2-aminothiazoles that demonstrated selective potency for the adriamycin-resistant and estrogen-negative breast cancer cell lines. The results suggest that these 2-aminothiazoles represent lead compounds for evaluation in animal models of breast cancer.