Around 20% of invasive breast carcinoma are oestrogen receptor alpha (ER) negative. Theoretically, this negativity could be either due to the result of downregulation of ER expression in the tumour cells, or the result of the tumour being derived from or differentiating towards cells which normally lack that expression. Normal basal, including myoepithelial, cells of the breast are ERnegative. CD10, smooth muscle actin and S100 are markers of these basal cells that can be used for their demonstration in routinely processed sections. This study was aimed at comparing the incidence of positivity for three myoepithelial markers in ER-negative and ER-positive invasive breast carcinoma. We have examined sections of 117 cases of breast carcinoma, including 77 ER-negative and 40 ER-positive cases, for the expression of CD10, smooth muscle actin and S100, using the avidin-biotin complex immunoperoxidase technique. A tumour was considered positive if more than 10% of the tumour cells were positively stained. In all, 36 (47%) ER-negative tumours were positive for one or more of these myoepithelial markers. The percentage of positively stained tumour cells varied between 30 and 100%. Of the 40 ER-positive tumours, only three (8%) were positive; two for S100 and one for actin, with none being positive for CD10. If cases stained only with S100 are excluded, as some of these may represent luminal differentiation, definite myoepithelial differentiation seems to be present in 29% (22/77) of ER-negative tumours as compared with 2.5% (1/40) of ER-positive tumours; a difference which is highly significant (P<0.001). It is suggested that at least 29% of ER-negative invasive breast carcinomas may be derived from or differentiating along the direction of basal nonconventional luminal epithelial breast cells that normally lack the expression of ER but totally or partially express various myoepithelial markers. Such tumours might need a different therapeutic approach.