The administration of cytotoxic agents followed by the administration of heat shock protein 90 inhibitors, such as ansamycins, has a synergistic effect on the growth inhibition of cells. This synergy occurs at doses of each cytotoxic agent that normally only causes minimal growth inhibition of cells. Such combination therapy thus allows one to use lower doses of cytotoxic agents to avoid or reduce their respective toxicity to patients without compromising their growth inhibitory effects. Thus, these combinations can be used for the treatment of an animal, preferably a mammal, that has a cell proliferative disorder, whether the cells have wild-type Rb or are Rb deficient or Rb negative. One such method, directed to treating cell proliferative disorders includes the step of administering a therapeutic effective amount of a cytotoxic agent followed by administering a therapeutic effective amount of a heat shock protein 90 inhibitor. The cytotoxic agent may be a microtubule-affecting agent, topoisomerase II inhibitor, a platinum complex, paclitaxel, or a paclitaxel derivative. The HSP90 inhibitor may be an ansamycin, radicicol or a synthetic compound that binds to the ATP-binding site of HSP90.