Retrovirus particles with type C morphology were found in two T-cell lymphoblastoid cell lines, HUT 102 and CTCL-3, and in fresh peripheral blood lymphocytes obtained from a patient with a cutaneous T-cell lymphoma (mycosis fungoides). The cell lines continuously produce these viruses, which are collectively referred to as HTLV, strain CR (HTLV_CR). Originally, the production of virus from HUT 102 cells required induction with 5-iodo-2′-deoxyuridine, but the cell line became a constitutive producer of virus at its 56th passage. Cell line CTCL-3 has been a constitutive producer of virus from its second passage in culture. Both mature and immature extracellular virus particles were seen in thin-section electron micrographs of fixed, pelleted cellular material; on occasion, typical type C budding virus particles were seen. No form of intracellular virus particle has been seen. Mature particles were 100–110 nm in diameter, consisted of an electron-dense core surrounded by an outer membrane separated by an electron-lucent region, banded at a density of 1.16 g/ml on a continuous 25–65% sucrose gradient, and contained 70S RNA and a DNA polymerase activity typical of viral reverse transcriptase (RT; RNA-dependent DNA nucleotidyltransferase). Under certain conditions of assay, HTLV_CR RT showed cation preference for Mg²⁺ over Mn²⁺, distinct from the characteristics of cellular DNA polymerases purified from human lymphocytes and the RT from most type C viruses. Antibodies to cellular DNA polymerase γ and antibodies against RT purified from several animal retroviruses failed to detectably interact with HTLV_CR RT under conditions that were positive for the respective homologous DNA polymerase, demonstrating a lack of close relationship of HTLV_CR RT to cellular DNA polymerases γ or RT of these viruses. Six major proteins, with sizes of approximately 10,000, 13,000, 19,000, 24,000, 42,000, and 52,000 daltons, were apparent when doubly banded, disrupted HTLV_CR particles were chromatographed on a NaDodSO₄/polyacrylamide gel. The number of these particle-associated proteins is consistent with the expected proteins of a retrovirus, but the sizes of some are distinct from those of most known retroviruses of the primate subgroups.