Human T-cell leukemia-lymphoma virus (HTLV) was first isolated from sporadic patients with adult T-cell malignancies in the United States and subsequently from T-lymphocytes established in culture from additional T-cell leukemia-lymphoma patients living in different geographical areas of the world. Co-cultivation of normal umbilical cord blood with lethally irradiated, HTLV-positive lymphocytes established in culture from many of these patients resulted in the productive infection of the cord blood T-lymphocytes which grew in suspension culture in the absence of exogenous TCGF. These transformed cord blood cells have morphological and cytochemical properties similar to HTLV-positive fresh and cultured tumor T-cells and are distinguishable from virus donor cells by HLA haplotype and chromosomal markers. These cells express HTLV proteins, release type-C virus particles and contain surface receptors for TCGF. These results demonstrate that HTLV isolated from T-cell leukemic donors from different parts of the world can productively infect and transform fresh human cord blood T-lymphocytes, and that the transformed cells share many similarities with fresh or cultured leukemic cells.