Enzyme immuno assay (EIA) of estrogen receptor (ER) has confirmed the results of earlier investigations using steroid binding techniques, namely that ER is present at very low concentrations in samples from metastatic melanoma. Thirty-four of 61 samples (56%) were ER positive with EIA. The corresponding figures using isoelectric focusing (IF) for the steroid binding assay were 16 of these 61 samples (26%). The difference between the methods may be due to difficulties in the interpretation of analytical results for IF at low ER concentration levels or to interference from other 3H-estradiol binding components. Estramustine binding site (EMBS) has been found in samples from 15 of 77 patients (20%) with IF in polyacrylamide gels. Estramustine is, together with estramustine, the cytotoxic metabolite of estramustine phosphate (Estracyt). In analogy to the previously suggested therapeutic significance of estramustine binding protein in the treatment of prostatic cancer, the clinical importance of estramustine phosphate should also be studied in metastatic malignant melanoma in correlation with EMBS status.