The thermolability of steroid hormone receptors was examined in a prospective study at the Gynaecology and Obstetrics Division, University of Erlangen, to clarify the cause of false negative hormone receptor findings. The loss of steroid hormone receptor content was measured in relation to the duration of operation, the time and duration of preservation. The investigations show a high thermolability of the steroid receptors. This is of considerable significance in conventional operations. Compared to the determination of hormone receptors at the time of biopsy for histological verification of the putative diagnosis of breast cancer, analysis at the end of the modified radical mastectomy and segment resection or excision of recurrences only detected 43% of the content of oestrogen receptors and 50% of the content of progesterone receptors. After storage of the surgical preparation for one hour at room temperature, the yield of oestrogen receptors fell to 35% and that of progesterone receptors fell to 31% of the original receptor content. A fresh determination of hormone receptors after cryopreservation for one to two weeks at -40 degrees C postoperatively showed only a minor loss of receptors amounting to 5.3% of the oestrogen receptors and 6.6% of progesterone receptors. On the other hand, cryopreservation at the same low minus temperature for five to six months, reduces the yield of oestrogen receptors by 34.6% and that of progesterone receptors by 33.9% compared to the intraoperative analysis.