Molecular clones of human T-cell leukemia virus type 1 (HTLV-1) have been constructed and stably propagated in plasmids in Escherichia coli. Expression of Tax could be demonstrated from these clones in fibroblast and epithelial cell lines. HOS cells stably transfected with HTLV-1 clone ACH produced all three classes of viral transcripts and Gag proteins. Virus-like particles were also produced from ACH transfected HOS cells as demonstrated by sucrose gradient and electron microscopic analyses. Transfection of peripheral blood mononuclear cells with ACH resulted in production of infectious virus particles which induced lymphocyte proliferation. This study describes useful reagents for further examination of the biological properties of HTLV-1.