An enzyme-linked immunosorbent assay (ELISA) using monoclonal and polyclonal antibodies against t-PA was used to measure the concentration of tissue-type plasminogen activator (t-PA) in plasma from 34 healthy donors and 92 breast cancer patients with a varying extent of disease. The mean value of t-PA in plasma for the healthy donors was 2.4 +/- 2.1 ng ml-1 (s.d.). The mean value for the breast cancer patients was 5.3 +/- 4.3 ng ml-1. This increase was statistically significant at the 1% level. There was a positive correlation between the mean t-PA plasma concentration and the extent of disease in different groups of patients. Taking 5.0 ng ml-1 as cut-off point, about 40% of the patients were positive, and 6% of the normal controls were false positive. Twenty-five per cent of the patients in complete remission, 28% of the patients with minimal tumour burden, 60% of the patients with moderate tumour burden, and 90% of the patients with massive tumour burden were positive. It is possible that the patients with an elevated plasma t-PA represent a group with a particularly bad prognosis.