Studies of various technics that aid in performing estrogen receptor assays on small tissue samples from primary cancers of the breast are reported. It is shown how sufficient protein can be obtained from less than 100 mg of small primary tumor without ultracentrifugation to assay estrogen receptor by a simplified two-point Scatchard plot method. The resulting Kd and maximum binding site values with 36 tumor tissue samples approximated the values obtained with the more laborious, larger tissue sample-demanding six-point Scatchard plot. Target and nontarget tissue controls are easily included in an assay run of four to eight samples. For interpretive purposes the derivation of the characterizing value of the Kd for estrogen receptor is given and compared with nonspecific associating binders.