The genetic and functional basis of the replication-defective nature of human immunodeficiency virus type 1 (HIV-1) in monkey cells was studied. By the generation and characterization of chimeras between HIV-1 and simian immunodeficiency virus, the sequence encompassing the 3′ half of the long terminal repeat, gag and pol genes of HIV-1 was found to be responsible for the growth restriction. Early and late phases of HIV-1 replication in monkey cells were analysed in detail using several assay systems: transfection/coculture, trans-complementation between various proviral clones carrying the CAT gene and effector clones and evaluation of transcription and reverse transcription. All the data were consistent with the notion that HIV-1 replication is blocked at a very early stage(s) such as uncoating and/or reverse transcription in monkey cells.