Forty patients with intermediate stage (T2 > 3 cm-T3, N0-N1) operable breast cancer received neoadjuvant chemotherapy by MCF (mitoxantrone, cyclophosphamide, 5-fluorouracil). Four cycles were administered at 3-week intervals. The obvious hematological toxicity (64% of grade III for the leucocytes and up to 34% of grade IV for the granulocytes) was rapidly reversible and did not hinder completion of the treatment. Ten patients showed a complete remission and a tumor volume regression of more than 50% was observed in 12 other patients. Tumor shrinkage allowed breast-saving surgery in 50% of the cases. A complete sterilisation of the surgical specimen was found in only two of the 40 patients and a few persisting neoplastic cells were found in ten other cases. A positive response at the level of the axillary lymph nodes was also obtained in more than 50% of the cases. In 25 of the 36 cases examined, the primary chemotherapy induced cellular lesions (fibrosis, necrosis) at the tumor level. A feasibility study was undertaken in order to determine quantitatively several biochemical parameters (steroid hormone receptors, cathepsin D, c-erbB-2 oncoprotein) in very small tumor samples obtained by Tru-Cut before any treatment and in surgical specimens. In the future, these micromethods will be used systematically with the aim of estimating the value of these potential prognostic factors for therapeutic follow-up of the patients.