Peripheral plasma concentrations of PAF were measured in patients with benign and malignant tumors of the breast, and in healthy female controls with a very sensitive PAF assay. The mean values+/-SEM were significantly higher in the malignant group than in healthy controls (1070+/-111 fmol/ml vs. 142+/-12 fmol/ml, P less than 0.01) and patients with benign tumors of the breast (1070+/-111 fmol/ml vs. 208+/-16 fmol/ml). Patients with malignant tumors of the breast and hypercalcemia contributed predominantly to the higher values of PAF (1500+/-167 fmol/l). The high levels of PAF were not found to be correlated to the clinical and histopathological data. The surgical removal of the tumor had little effect on the plasma concentration of PAF. In contrast, hypercalcemic patients showed an increase, although not significant, in plasma PAF levels after the surgical operation.