Mononuclear phagocytes, an integral part of the lymphoreticular infiltrate of human and experimental tumors, might contribute to fibrin deposition within malignant tissues through the production of procoagulant activity (PCA). Recent studies in different types of experimental tumors and in some cancer patients indicate that the functional status in terms of PCA of mononuclear phagocytes at 'peripheral' sites is not the same as that of macrophages at the tumor site (tumor-associated macrophages, TAM). Peripheral blood monocytes and/or peritoneal macrophages from V2-carcinoma-bearing rabbits and from mice bearing different types of tumors, like cells from normal animals, have low levels of basal PCA and are able to respond with a marked increase in PCA to in vitro stimulation. Likewise, in patients with primary lung cancer, pulmonary alveolar macrophages (PAM) obtained from the contralateral side of the neoplasm and blood monocytes behave essentially as cells from normal individuals. In contrast, profound changes in PCA have been found in TAM. In some experimental tumors and in a number of the patients, macrophages taken at the tumor site express high levels of basal PCA, which most probably reflect in vivo activation. Local activation of macrophages for PCA production might contribute to fibrin deposition at the tumor site. In the other experimental tumors studied and in the remaining patients, not only have macrophages low basal PCA but they also fail to respond to various stimulants in vitro. Although the mechanisms underlying these changes and their pathophysiological significance remain to be established, it is apparent that neoplastic growth may modulate the expression of macrophage PCA at the tumor site.(ABSTRACT TRUNCATED AT 250 WORDS)