We previously reported that 7,12-dimethylbenz(a)anthracene (DMBA) competes with progesterone for progesterone serum carrier protein(s) and target tissue receptors. In addition, we established that DMBA is a mild progestational agent. DMBA administration increases the uterine weight and induces endometrial glandular differentiation in estrogen primed rabbit. Additionally. DMBA treatment reduced uterine estrogen receptor level to below the detectable level on day 20 and there was a three-fold increase in progesterone receptor level. DMBA injected into mammary pads of rats and rabbits induced hyperplasia in stromal cells following 10 days of treatment and increased the progesterone receptor levels, while there was no change in estrogen receptor levels. Attempts to separate those hyperplastic cells from other mammary cells based on size and density using albumin gradient centrifugation was not successful. In order to detect whether estrogen receptors are present in those hyperplastic cells induced by DMBA, a specific fluorescent probe was developed by conjugating fluorescein to estrogen. We are now able to detect cells containing estrogen receptors using the fluorescein conjugated estrogen and will use this technique to detect cells containing estrogen receptors. We plan to use a similar fluorescent probe to detect progesterone receptors. The fluorescent probe technique may provide a means to detect those cells in which there is either an increase or decrease in estrogen and progesterone receptors.