When human breast cancer-derived MCF-7 cells were maintained in low estrogen medium (phenol red-free), the cells adapted to grow without added estrogen, but growth could be inhibited by antiestrogen in the medium. Estrogen-stimulated progesterone receptor levels remained basal but could be stimulated by estradiol. Estrogen receptor content increased steadily during adaptation, which may model the increasing levels of estrogen receptor observed in breast cancer with increasing patient age. The mechanism of the adaptation to low estrogen medium is unclear; however, cell lines such as MCF-7 may need to be cultured in the presence of an estrogen such as phenol red in order to maintain a stable estrogen-sensitive phenotype. On the other hand, maintenance of estrogen-dependent cells in low estrogen media may convert them to dependence on factors which are not currently understood. This may ultimately increase their value as models of hormone action.