In-frame mutations were introduced into various portions of the human immunodeficiency virus type 1 (HIV-1) gag gene, and potentials of the mutants to suppress the replication of wild-type HIV-1 were monitored. In contrast to results obtained with matrix and nucleocapsid mutants, almost all capsid mutants blocked HIV-1 replication completely in single-round replication assays. A capsid mutant designated C6b was demonstrated to be one of the most efficient inhibitors for HIV-1 reported to date, and to be effective at both early and late viral replication phases. T-cells, which are engineered to express the C6b Gag in response to HIV-1 infection, were perfectly resistant to HIV-1. (C) 1997 Federation of European Biochemical Societies.