A general cancer-associated SCM factor has been isolated, purified to homogeneity, and characterized, and methods for its use have been described. The SCM factor is a low molecular weight peptide able to pass through a filter with a nominal molecular weight cutoff of 1000 daltons, but not through a filter with a nominal molecular weight cutoff of 500 daltons. The filter has the approximate amino acid composition of (Asx2, Glx3, Ser, His, Gly5, Thr, Arg, Ala3, Tyr, Met, Val3, Phe3, Ile, Leu3, Lys2), and has the ability to produce at least a 10 % decrease in the intracellular fluorescein fluorescence polarization value of potentially SCM-responding lymphocytes from blood samples of donors with cancer. Tryptic peptides of fifteen and sixteen amino acids not including the amino terminus of the SCM factor have also been purified, and have SCM activity. The SCM factor can modify the SCM response of lymphocytes from donors free of cancer to the response characteristic of lymphocytes from donors afflicted with cancer. The factor can also diminish the natural in vitro cytotoxicity of killer lymphocytes toward tumor cells. A method for purifying the SCM factor from blood is described, leading to a factor purified to substantial homogeneity by reverse-phase high pressure liquid chromatography. The SCM factor is useful for screening of blood samples for the presence of malignancy in the donor by the SCM test. Methods for reducing in vivo activity of SCM factor, such as dialysis or antibody neutralization, can also be useful in the management of cancer.