More robust envelope specific lymphoproliferation is induced in mice immunized with CD4bs- Env as compared to CD4bs+ Env. (A) Mice were immunized four times i.p. with CD4bs+ Env, CD4bs- Env, or no antigen in the presence of incomplete Freund's adjuvant. (B) Mice were immunized five times i.p. with the same envelope antigens and Ribi adjuvant. Splenocytes were collected two weeks after the final boost and tested for envelope-specific proliferation by 3H-thymidine incorporation. Proliferation in the presence of antigen (3 μg/ml) or medium alone was measured after a 5-day culture. The stimulation indices of individual mice and the mean values for each group (horizontal bars) are shown. Statistical analyses were done using one-way ANOVA with Bonferroni's multiple comparison test.