The mammary carcinoma cell line MFM-223 is characterized by high androgen and low estrogen and progesterone receptor levels. With the dextran charcoal method, androgen binding was determined at 160 fmol/mg protein corresponding to approximately 100,000 binding sites per cell in whole cell binding assays. The estrogen and progesterone receptor contents were between 8 and 18 fmol/mg protein. The proliferation of MFM-223 cells was significantly inhibited by doses greater than 0.01 nM dihydrotestosterone. The androgenic inhibition of cell proliferation was antagonized by the antiandrogens cyproterone acetate and hydroxyflutamide. In spite of the low estrogen receptor content, MFM-223 cell proliferation was slightly enhanced by 10 nM 17 beta-estradiol. Treatment with 17 beta-estradiol or dihydrotestosterone failed to provoke an increase of the progesterone receptor level. MFM-223 cells have characteristic patterns of isoenzyme polymorphism and of karyotype alterations revealing marker chromosomes and homogeneously staining regions. In the spectrum of human mammary carcinoma cell lines, MFM-223 cells offer a unique model to investigate molecular mechanisms of androgen receptor action.