The tandem repeat peptide of the circumsporozoite protein of Plasmodium cynomolgi, (NAGG)5, conjugated to BSA or Salmonella flagella, was injected into mice with block copolymer and other adjuvants. The flagella carrier preferentially stimulated IgG2a antibodies to (NAGG)5 that constituted as much as 85% of the total IgG antibody whereas the BSA carrier stimulated as much as 98% IgG1. The distribution of isotypes of antibody to (NAGG)5 was also modified by using the copolymer adjuvants L121 or L141, either alone, or especially in combination with a nontoxic LPS. L121 or L141 increased the proportion of IgG2a and IgG2b antibodies to (NAGG)5 after immunization with (NAGG)5-BSA whereas LPS stimulated further increases in IgG2 antibodies (up to 69% of the total IgG). The hapten density, physical form of emulsion, and route of immunization further affected the isotypes produced in this study.