Vaccines may provide a route of therapy against HIV-1 infections by boosting the immune system responses. An artificially constructed HIV-1 vaccine (NYCBH), using vaccinia virus as its vector, has the advantage of conferring both cellular and humoral immune responses that are long-lived. Studies have shown that a second artificially constructed vector vaccine, HIV-1 canarypox (vCP205), also increases CD8+ cytotoxic T lymphocyte (CTL) activity, a cell-mediated immune response. Yet, immune responses are not boosted in volunteers previously vaccinated with vaccinia-based HIV-1 vaccines when a second vaccination with the same vaccine is given. One theory for vaccinia vaccine's failure is that immunologic barriers by antibodies to the vector itself may be responsible. This study examines the effectiveness of boosting the immune responses following vCP205 vaccination in the following: 1) volunteers who were previously immunized with vCP205 vaccine who may or may not have shown increased immune responses following the first immunization, and 2) volunteers who were previously immunized with NYCBH vaccine.