The proteins of a highly leukaemogenic murine virus (DMBA-LV) endogenous to the CFW/D mouse have been characterized. This virus does not contain ecotropic, xenotropic or polytropic type C retroviruses capable of replicating in established tissue culture cell lines. Analysis of the viral proteins indicated that proteins characteristic of both type C and type B retroviruses were present. All the proteins characteristic of a competent, mature type B retrovirus, such as the murine mammary tumour virus, were present in the virions and were expressed at high levels in a virus-producing thymic lymphoma cell line. Type C internal structural proteins were detected in the virus and cells producing virus. Type C envelope protein was detected at very low levels in both the virions and the virus-producing tumour cell lines. Chymotryptic polypeptide profiles of this gp70 indicated similarity with the envelope glycoproteins characteristic of xenotropic and recombinant type C retroviruses.