The effect of a C-nucleoside [4-(beta-D-erythrofuranosyl)-imidazoline-2-thione] on cell proliferation was studied. Two types of cell proliferation were used: Ehrlich ascites tumor cells obtained from NMRI mice, and the proliferative response of T lymphocytes from human peripheral blood to a mitogen (phytohemagglutinin). The uptake of 3H-thymidine was used as an index of cellular growth. The results indicate that this C-nucleoside had an inhibitory effect on both types of cell proliferation studied. Its possible application in tumor therapy and as an immunosuppressor in transplants is discussed.