Studies were undertaken to test the female sex hormone β‐estradiol for its ability to modify lethality in young adult female and male mice exposed to lead acetate. Mice were injected with β‐estradiol on d —I and 0, and immediately after the second injection lead acetate was administered at four subgroup dosages ranging from 75 to 600 mg/kg body weight. Control mice were injected with only lead acetate. On d 4, 6, and 8 after Pb, the median lethal dosages (LD50s) were calculated from the observed mortality ratios by a method of moving averages. LD50 values were considerably lower for the Pb‐ and β‐estradiol‐injected groups than for the controls. Data from these experiments indicate that the β‐estradiol treatments render both female and male mice more vulnerable to the lethal effects of lead acetate.