We report the complete nucleotide sequence of the potent T-lymphomagenic murine retrovirus SL3-3. The non-LTR regions of the virus show 98% sequence identity to the endogenous ecotropic Akv murine leukemia virus. While the region encoding the surface envelope protein is completely identical to that of Akv, a approximately 200 nucleotide stretch in the integrase encoding region upstream of env is similar to the sequence of mink cell focus forming (MCF) viruses and shows a complete match with the mouse retrovirus 10A1. The history of SL3-3 may therefore include recombination involving an Akv-like virus and a virus with MCF-like sequences.