The P component of amyloid is a normal serum protein designated SAP. SAP has substantial homology with C-reactive protein (CRP). However, unlike CRP, SAP is not an acute-phase reactant in man. Recent studies have established SAP as a major acute-phase protein in mice. Moreover, mice which have received tumour implants have also been found to have raised serum concentrations of SAP. The aim of the present study was to determine possible association between the serum level of SAP and human cancer. We found that patients with carcinoma of the breast have significantly increased serum concentrations of SAP. Moreover, in these patients SAP levels correlated with the severity of the disease. Patients with carcinoma of the colon, however, did not differ from healthy individuals in the serum level of SAP. Possible explanations for this discrepancy are discussed.