The effects of triptorelin (Decapeptyl) and heparin, alone or in combination, on the intracellular concentration of estradiol (E2) after incubation of estrone sulfate (E1S) with the MCF-7 mammary cancer cells was investigated. Although heparin (10 mg/l culture medium) or Decapeptyl (5 x 10(-7) or 5 x 10(-5) mol/l) did not affect the levels of radioactive E2 after incubation with [3H]E1S, the combination of Decapeptyl and heparin significantly decreased the radioactive uptake and the intracellular concentrations of [3H]E2. In the absence of Decapeptyl and heparin incubations with E1S, the resulting E2 concentration (in nmol/kg DNA +/- SD) was 558 +/- 44; after heparin (10 mg/l) plus Decapeptyl (5 x 10(-7) or 5 x 10(-5) mol/l) the values were 389 +/- 55 and 165 +/- 18, respectively. It is concluded that Decapeptyl with heparin can decrease very significantly the conversion of E1S to E2 in MCF-7 cells, an observation that suggests new possibilities for the control of E2 in hormone-dependent breast cancer.