AbstractEstramustine, a conjugate of estradiol and nitrogen mustard, has been shown previously to depolymerize microtubules, inhibiting the formation of mitotic spindles and arresting cells in G2/M. Estramustine at concentrations of 10–30 μM inhibited the growth of EMT6 mouse breast carcinoma cells in a concentration‐dependent fashion. The viability (clonogenicity) of the cells also decreased as the drug concentration and exposure time increased. Pretreatment of EMT6 cells for 48 hr with 10 or 20 μM estramustine increased the radiosensitivity on EMT6 cells, producing enhancement ratios of 1.2 and 2.1, respectively. © 1995 Wiley‐L