Constructs were generated in which expression of human immunodeficiency virus type 1 (HIV-1) protease (PR) was placed under control of the HIV-1 long terminal repeat, thus requiring the HIV-1 Tat protein for expression of PR. The activity of PR was assessed by cotransfection with a construct producing a Gag substrate. Expression of PR as an intramolecular multimer resulted in a large increase in PR activity in comparison with the level obtained with the expression of PR as a monomer. A cytotoxic effect of PR expression was also exhibited by the constructs expressing PR multimers. CD4+ T-cell lines were generated with a construct producing PR as a linked tetramer and screened for PR activity and inducibility. The replication of HIV-1 in these cell lines was several orders of magnitude reduced in comparison with that in cell lines not expressing PR. Infection in these cell lines could be detected early after infection but disappeared over time. Infection of the PR-expressing cell lines could be increased several orders of magnitude by the addition of a specific inhibitor of PR, U75875 (Upjohn), after infection of the cells, demonstrating that the potent inhibition of HIV-1 replication in these cells was directly due to the expression of PR.