MCF-7 cells are a slow growing estrogen receptor (ER) positive human breast cancer cell line that is commonly used to model estrogen responsive breast cancer cell growth in-vitro and tumour growth in-vivo. These tumours require estrogen supplementation, and in-vivo doses of between 0.72mg and 2mg estradiol pellets are commonly implanted in the dorsal flank of ovariectomised, immunocompromised mice. We wanted to grow MCF-7 tumours in immunocompromised mice without the need to be ovariectomised. When we treated immunocompromised mice with 0.72mg pellets to induce MCF7 tumour growth, the mice developed urosepsis. We have now shown that lower doses of estradiol pellets, 0.3mg and 0.5mg, induce elevated serum estrogen levels and maintain tumour growth, without causing urosepsis. Supplementation for only one week did not support sustained MCF7 tumour growth. In conclusion, 0.3mg and 0.5mg silastic pellets can be used to stimulate ER+ breast cancer growth in ovary-intact, immune compromised mice.