An effective vaccine against infection with human immunodeficiency virus type 1 (HIV-1) is thought likely to require both a humoral and a CTL immune response. A non-replicating adenovirus vector system has been developed that can induce both a humoral and CTL response to HIV-1 envelope in mice. It is demonstrated that the stimulatory tat/rev 5' splice-donor site sequence is required for efficient expression of HIV-1 env by this adenovirus vector system. rev can be provided bicistronically or in trans to result in good expression of env in vitro. A humoral immune response was detected after two immunizations with a bicistronic recombinant adenovirus (RAd142). The response was dose dependent, 5x10(7) p.f.u. inducing a response in some, but not all, animals and 1x10(8) p.f.u. giving a consistent antibody response. However, CTLs were induced by the lower dose of virus and after only one immunization with the higher dose. A positive CTL response was also seen consistently when the two monocistronic adenoviruses (RAd501 expressing env and RAd46 expressing rev) were given together, although two immunizations were required to give approximately the same level of response as seen with the bicistronic virus. RAd501 on its own also gave a low CTL response when two immunizations were given. It is suggested that a lower level of env expression is required to produce a CTL response than a humoral response and that this nonreplicating adenovirus vector is a good system for inducing CTL.