The immunogenicity of a yeast-expressed 19 kDa fragment of P vivax MSP-1 in the presence of different adjuvant formulations was evaluated. ICR mice were immunized with the 19 kDa antigen, using Freund's, alum, and block copolymer P1005 in water-in-oil (W/O) or oil-in-water (O/W) emulsions with or without detoxified lipopolysaccharide (RaLPS) as adjuvants. Five weeks following immunization with the antigen, mice were boosted with asexual blood-stage antigens. Three weeks after the last immunization with the 19 kDa antigen, mice from the Freund's group and most groups that received P1005 as adjuvant had higher total IgG titres than those that received alum as adjuvant or antigen alone. Antibody responses after the antigen immunization were predominantly of the IgG1 isotype, but mice in the Freund's and P1005 (W/O or O/W emulsion with or without RaLPS) groups also had high titres of IgG2a and IgG2b. Antibody titres against merozoites increased in all groups after the parasite antigen boost. IgG2a levels in the group that received antigen in P1005 plus RaLPS in the W/O emulsion were higher than those receiving Freund's, alum or the other copolymer adjuvants. The high IgG2a titres in this group were associated with reduced IL-10 production.