Various correlations have been made between prolactin (PRL) receptor positivity and different physiological and pathological conditions. A number of reports concluded that high serum PRL levels in patients with advanced breast cancer are associated with a relative inefficacy of either endocrine or cytotoxic therapy, whereas a reduction of high PRL levels was followed by an increased response to cytotoxic therapy. The possible correlation between PRL receptor-positivity and the presence of steroid receptors has been studied by different authors with different conclusions. Since tumoral tissue specimens are obtained in surgery, it was suspected that the rise in PRL plasma levels which occurs during general anesthesia might block the available receptor sites. Many human breast cancer cell lines in long-term tissue cultures possess varying amounts of membrane receptors for PRL. Some receptor-positive cell lines appeared responsive to human PRL in vitro and an up and down regulation of PRL binding was observed after pretreatment with subphysiological or higher human PRL concentrations, respectively.