To evaluate whether or not the finding of platelet activation in patients with tumors is related to the stage of malignancy, a study of biochemical markers indicative of the presence of circulating activated ('exhausted') platelets was carried out in 95 untreated patients with breast adenocarcinoma or malignant melanoma, localized or spread to regional lymph nodes with no detectable distant metastasis. These tumors were chosen as examples of tumors which can be accurately staged for localization or spread, and as examples of mucin-secreting tumors (breast adenocarcinoma) or neuroectodermic tumors (malignant melanoma). Results were compared with those for 26 patients with benign breast disease, 23 blood donors and 50 hospital workers. The most frequent abnormalities were low levels of intraplatelet ADP and 5-hydroxytryptamine and high ATP/ADP ratios. Although these abnormalities occurred with both types of tumor, they were more frequent and marked for melanomas and breast carcinomas spread to regional lymph nodes. Our data indicate that the presence of exhausted platelets is an early finding in patients with malignant tumors.