A study of 378 patients with infiltrating breast carcinoma using linear logistic regression and ANOVA analysis demonstrated a different relation between age at operation and estrogen-receptor (ER) concentration in the lymph-node negative and the lymph-node positive groups. Tumours from patients between 51 and 70 years old had lower median ER concentration in the lymph-node negative group than in the lymph-node positive group. In the group older than 70 years, however, tumours from lymph-node negative patients had higher median ER concentrations than those from the lymph-node positive patients. Patients 50 years and younger had similar median ER concentrations in both lymph-node groups. Low mean nuclear area (MNA) of the tumour cells was associated with high frequency of tumours able to produce ER. No such association was found for age. Independent of age and lymph-node status tumours with low MNA also had high ER concentration. These findings suggest that tumours from different lymph-node/age groups may have different biological properties. The relationship between ER and nuclear size point to a key function of the nucleus, both as regards the ability to produce ER and its level of production.