Estrogen and progesterone receptors (ERs and PRs, respectively) were measured in both cancerous and noncancerous components of 104 modified radical mastectomy specimens. In addition, ER and PR levels were determined for 14 benign breast specimens obtained by reduction mammoplasty. The receptor levels were measured by scatchard method. Each of these groups - cancerous, corresponding noncancerous, and mammoplasty specimens - were divided into subgroups according to their receptor levels. Fourteen of the 104 noncancerous specimens were found to be ER positive (ER+). Most cases of ER+ noncancerous tissue (13 of 14 cases) were associated with ER+ tumors. The reverse was not true because only 13 of 64 cases of the ER+ tumors were associated with positive ER in their noncancerous counterparts. Comparable results were obtained for PR. The average ER-PR level of the noncancerous mastectomy tissue was significantly higher than that of the mammoplasty specimens despite the similar histologic findings in both groups.