The determination of progesterone receptors (RP) was performed on 80 benign and malignant human breast tumors with a single saturating dose method (10 nM) using dextran-coated charcoal (PR-Bio) and an enzymo-immunoassay (PgR-EIA). There was a significant correlation between the 2 methods qualitatively (P less than 0.001) and quantitatively (r = 0.79). However the results were significantly higher using the PgR-EIA method than the PR-Bio method (P = 0.04) with a regression line Y = 0.81 x +0.58.