The effect of various polycations on the immune response potentiated with poly I:C was studied. It was found that low molecular weight polycations had no potentiating effect. Polylysine was ineffective whereas protamine was superior to lysozyme, poly-arginine, poly-histidine, DEAE-Dextran and histone. A foot-and-mouth disease trivalent vaccine composed of strains A24 Cruzeiro, O1 Caseros and C2 Resende elicited no immune response in swine when adjuvanted with aluminium hydroxide but was effective when emulsified in oil. In general, the immune response was potentiated ten-fold when the emulsion contained poly I:C. The antibody production was in most cases further potentiated by a factor of ten when the nucleic acid double-strand was complexed with 1 : 10 (w/w) DEAE-Dextran. Protamine was as effective, or perhaps even more, than DEAE-Dextran to this effect. Guinea pigs vaccinated with a water-in-oil emulsion type monovalent C3 vaccine showed an increase in antibody production when the vaccine contained poly I:C or poly I:C complexed with 1 : 10 (w/w) protamine.