Fifty-two invasive ductal carcinomas of the breast were evaluated immunohistologically with a panel of monoclonal antibodies to study the frequency distribution and the localisation of the tumor-infiltrating lymphoreticular cells. The analysis yielded the following results: 1) The lymphoreticular cells mostly accumulated in the intervening and surrounding stroma while the tumor foci regularly exhibited a considerably lower degree of infiltration. 2) The main components of the cellular stromal reaction were monocytes/macrophages, occurring in high numbers in more than 80%, and T4 cells, which were observed in high numbers in 60% of all analyzed tumors. 3) While 2/3 of all cases showed moderate numbers of T8 lymphocytes, the B lymphocytes and natural killer cells generally were encountered in very low numbers or were nearly absent from the lymphoreticular infiltrates. Conclusions on the functional significance of the tumor-infiltrating cells cannot be drawn from an in situ histological study alone, but the preferential intrastromal accumulation of most cells seems not to be indicative of an intensive host defense against clinically detectable human breast cancers.