Fifty-two invasive ductal breast cancers were investigated histologically and immunohistologically to assess localization and composition of the lymphoreticular infiltrates. The tumour-infiltrating cells were mainly located in the intervening stroma, whereas tumour foci often exhibited lower numbers of lymphoreticular cells. Macrophages (Mono 1+ and KiM 6+) and helper/inducer cells bearing the T4 surface antigen (Leu-3a+) regularly constituted the majority of the tumour-infiltrating lymphoreticular cells. In more than 80% of cases large numbers of macrophages were found, and many T4 cells occurred in about 60%. Next in frequency were the T lymphocytes (Leu-1+) which were mostly observed in high (46%), or in moderate (39%) numbers. In about 2/3 of the cases moderate numbers of T8 (suppressor/cytotoxic) lymphocytes (Leu-2a+) were detected. B lymphocytes (T0 15+) and natural killer cells (Leu-7+) were generally encountered in very low numbers, while eosinophilic granulocytes were virtually absent from the lymphoreticular infiltrates. Tissue mast cells and plasma cells were present in very low numbers in about one half of the tumours but cases with low, moderate or - rarely - even high numbers of infiltrating cells also occurred. It must be emphasized that an in situ histomorphological analysis of the cellular part of the stromal reaction of invasive ductal breast cancers allows only limited conclusions concerning the functional properties of the tumour-infiltrating lymphoreticular cells. From the present study, macrophages and T4 cells but also T8 lymphocytes might be of significance in immunooncological reactions "against" clinically detectable stages of invasive breast cancer.