Both soluble and nuclear oestrogen receptors were measured in at least two different portions of primary breast cancer and in concurrent metastatic tissue from axillary nodes. Oestrogen receptor (ER) status of involved nodes was found highly consistent with that of primary tumours. Of the 67 patients studied, 30 had metastatic nodes which contained both soluble and nuclear ER. Of these, 27 were associated with a primary cancer which also had both soluble and nuclear ER, determined in at least two separate parts of the primary cancer. Conversely, none of the completely negative primaries gave rise to fully receptor positive metastatic tissue. Surprisingly, 17 out of 20 heterogeneous primary tumours, i.e. those containing both receptor positive and negative components, generated receptor negative metastatic nodes. Moreover, in 7 of the 8 patients with N-2 stage nodal involvement, the metastatic disease had arisen from primaries which were either completely receptor negative or with a heterogeneous ER status. It is suggested that macroscopic heterogeneity of ER status in primary breast cancer is associated with poor prognosis.