Previously, we described a protective immune response induced by the carboxyl-terminal region of the merozoite surface protein-1 (MSP-1) from the rodent malarial parasite Plasmodium yoelii yoelii 17XL, expressed as a fusion protein and designated glutathione S-transferase (GST)-PYC2. We also demonstrated that the humoral response induced by GST-PYC2 was the primary mechanism by which immunized animals controlled their blood-stage infections. We have now examined the influence of several adjuvants on the immune response to the GST-PYC2 fusion protein. While alum, Freund's adjuvant, Ribi adjuvant system, and TiterMax were efficacious in eliciting a protective response with GST-PYC2 in BALB/c mice, saponin failed to induce protection, although significant levels of PYC2-specific antibodies were produced in all immunized animals. This protection depended on the mouse strain since immunization of Swiss Webster mice with GST-PYC2 in alum did not produce levels of PYC2-specific antibodies comparable to those in BALB/c mice nor did it induce any demonstrable level of protection against parasite challenge. Swiss Webster mice were protected, however, when immunized with GST-PYC2 in other adjuvants. Immunization with PYC2, isolated free of GST induced lower levels of antigen-specific antibody; only those animals given PYC2 in Freund's adjuvant demonstrated a significant degree of protection, suggesting the possibility (of additional cellular effector mechanisms. These findings demonstrate that adjuvant, host genotype, and the fine specificity of the response significantly influence the protection induced by the carboxyl terminus of MSP-1 in vivo and illustrate the need to consider these factors in evaluating MSP-1 as a vaccine component.