Guinea pigs (groups of 4) were immunized 4 times at 4-week intervals with HA-C14S/gp41 DNA (Group 1, 200 µg DNA per guinea pig per dose), HA-C14S/gp41 VLP (Group 2, 100 µg VLP per guinea pig per dose), SHIV 89.6 VLP (Group 4, 100 µg VLP per guinea pig per dose), or SIV Gag VLP (Group 4, 100 µg VLP per guinea pig per dose) as indicated, and blood samples were collected at 2 weeks after the second and fourth immunizations (A). The levels of antibody responses against HIV gp41 (B), HIV gp120 (C), or influenza HA (D) in sera from individual guinea pigs after 4 immunizations were determined by ELISA as described in Materials and Methods, and were expressed by the O.D. values at indicated serum dilutions for each antigen.