The chemotactic responsiveness of peripheral blood monocytes was measured in 194 individuals: 37 patients with breast cancer, 17 patients with a history of breast cancer but clinically free of disease after surgery, 42 patients with benign breast masses, and 98 normal controls. Monocyte chemotactic responsiveness (MCR) in vitro was not significantly different from normal [mean = 72.8 migrating monocytes/oil immersion field, +/- 9.3 (1 SD)] in 2 groups of patients: a) those with benign breast masses (mean = 72.6 +/- 15.1; P greater than 0.3) and b) those previously having breast cancer resected and remaining clinically free of disease (mean = 69.0 +/- 12.5; P greater than 0.4). However, MCR was significantly depressed in the group of patients with active breast cancer (mean = 57.2 +/- 20.7; P less than 0.0025). Resection of malignant breast masses resulted in a significant change in MCR (P less than 0.0025), whereas resection of benign lesions did not (P greater than 0.4). MCR was abnormal in all clinical stages of breast cancer, including breast cancer without evidence of metastasis to regional lymph nodes. These data supported the hypothesis that neoplasms adversely affect monocyte function and may thereby hinder immunologically mediated destruction of malignant cells.