Abstract In the Kansas Breast Study, the presence of circulating antibody to breast cancer antigen(s) has been demonstrated in 35 per cent of patients with carcinoma of the breast. Histopathologic studies have shown a positive correlation between those with positive antibody and the degree of lymphoid infiltration in the tumor and sinus histiocytosis in the lymph nodes. Both of these features are associated with an improved prognosis in patients with breast cancer. This study supports theories relating these histologic features to host-tumor defense mechanisms initiated by the lymphatic-reticuloen-dothelial system. Already, within three years, there is evidence to suggest that the presence of circulating antibody is associated with delay in recurrence of disease, as shown in those patients who had positive nodes at the time of surgery.