Immunosuppression observed in patients with malignancy may be due to factors released by tumour cells. Medium conditioned by COLO 205 cells was found to inhibit mitogen-stimulated lymphocyte proliferation. Examination of CD25 and Class II MHC induction on PBMC incubated in complete, or COLO 205 conditioned, medium was not significant. The prevalence of lymphocytes in the S-phase of the cell cycle was enhanced after mitogenic stimulation and addition of COLO 205 conditioned medium. This was balanced by a concomitant fall in proportion of cells in the G0/G1 phases of the cell cycle. The immunosuppressive properties of COLO 205 conditioned medium was abrogated by heating to 60 degrees C for 30 min and by digestion with trypsin. Fractionation of the medium by gel filtration yielded two immunosuppressive fractions with relative molecular weights of 60,000 and below 20,000. It was concluded that cultured COLO 205 cells produce immunosuppressive protein/peptide factors which block cell proliferation during DNA synthesis. These factors fail to prevent upregulation of membrane-associated markers of cell activation.