In 101 breast cancer patients, measurement of oestrogen receptor status in multiple biopsies across a tumour reveals a highly significant difference in the proportion of patients remaining either disease-free (P less than 0.04) or alive (P less than 0.005), when those with uniformly receptor positive (++) primary tumours are matched with clinically comparable patients whose tumours were homogeneously receptor negative (--). Mean follow-up time was 85 months. The prognostic value of this discriminant is particularly striking in the 53 patients with involved nodes at presentation. Of these, 13 were (++) and seven remain alive of whom six are disease-free, whereas 24 of the 29 (--) patients are dead. These results further suggest that receptor assay on a single homogenate gives less clinical information than do assays on multiple biopsies across the tumour. For patients with involved nodes, clinical management may best be decided after determination of 'macroheterogeneity'.