Retroviruses have been isolated from many animal species and several have been shown to be the aetiological agents of naturally occurring leukaemias, lymphomas and sarcomas (for recent reviews see ref. 1). There is evidence that these viruses or components of them are present in humans, but the isolation and analysis of retroviruses from human cells and evidence for an association with disease have been difficult to demonstrate (see refs 2,3). We now report the isolation and characterization of a new type C retrovirus (HTLV_MB) from a patient with a cutaneous T-cell leukaemia (Sézary syndrome). Analysis of the proteins and nucleic acids of this retrovirus isolate indicates that it is closely related to HTLV_CR, a retrovirus recently isolated⁴ and characterized^5–7 from a patient with cutaneous T-cell lymphoma (mycosis fungoides) but distinct from known animal retroviruses. Proteins and nucleic acids of HTLV were also identified in the fresh (uncultured) Sézary leukaemic blood cells from which the second isolate was derived.