The expression of transferrin and epidermal growth factor receptors in breast carcinomas, as detected by immunohistochemistry, has been compared with DNA ploidy and S-phase content of the same tumours as determined by DNA flow cytometry of fixed, paraffin embedded tissue, and with tumour differentiation. Good correlations have been found between DNA ploidy, S-phase content and differentiation. The expression of transferrin receptor appears to relate to DNA ploidy, but only to a limited extent to S-phase content, indicating that in breast carcinomas immunoreactive transferrin receptor may not be directly related to proliferation. The presence of epidermal growth factor receptor shows a close correlation with high S-phase content, but only a lesser association with DNA ploidy, and no correlation with tumour differentiation. DNA flow cytometry provides a more reliable method of assessing the relationship of such growth factor receptors in breast carcinomas to cell proliferation, than that obtained from more subjective tumour grading criteria.