PCEP enhances IgG1 and IgG2a X:31-specific serum titers when PCEP+X:31 is administered through intranasal, subcutaneous, and oral routes, but not after intrarectal immunization. BALB/c mice were given PCEP+X:31 (closed symbols) or X:31 (open symbols) through IN (A, E, squares), SC (B, F, circles), oral (C, G, triangles) and IR (D, H, diamonds) immunization. X:31-specific IgG1 (A-D) and IgG2a (E-H) were assayed in mouse serum. Groups given PCEP+X:31 with different letters are significantly different from each other within each IgG subtype (p < 0.05). A single asterisk indicates an adjuvant effect of PCEP during the course of the experiment, while a double asterisk indicates an adjuvant effect only at the specified time point. Arrows indicate the time of delivery of a secondary immunization using the same delivery route as the primary immunization.