Dextran 70 ('Lomodex') roduced significant increases in thymidine incorporation in a modified whole-blood method of investigating lymphocyte blastogenesis. This blastogenesis occurred at molar concentrations of 7 X 10(-6) and 7 10(-5). At 7 X 10(-7) mol/l dextran alone did not significantly increase thymidine incorporation; however, when this concentration of dextran was added simultaneously with phytohaemagglutinin there was a significant increase in incorporation. Conversely when dextran was added before P.H.A., thymidine incorporation was decreased. These variable effects indicate that dextran 70 may be a B-cell mitogen. Dextran 70 is widely used and these effects on lymphocyte blastogenesis may be clinically relevant.