The ultrastructure of 47 primary breast cancers was studied. For each tumour, the characteristics of approximately 200 malignant cells were examined. Eleven features were scored from 1 to 3 and multivariate analysis showed that seven of these could be used to define an ultrastructural index of differentiation. Differentiation at the ultrastructural level was associated with the presence of steroid receptors. Differentiated tumoral cells contained oestrogen receptor and progesterone receptor in 81.8% and 66.7% of cases, respectively. Poorly differentiated cells contained oestrogen receptors in 50% and progesterone receptor in 14.3% of cases. Comparison of histological grading by the method described by Scarff, Bloom & Richardson with the ultrastructural index of differentiation showed a rather loose correlation which was not significant in this group of 47 patients. The authors conclude that differentiation at tissue or cellular levels yields differing information, only the latter being closely correlated with steroid receptor presence.